The Class A amplifier - build and test (2/2)
Summary
TLDRIn this educational video, the creator discusses designing, simulating, and building a single-transistor wideband common emitter amplifier. The goal is to deliver half a watt into a 50-ohm load using a BD135 NPN transistor. The video covers the calculation of components, biasing, and efficiency, followed by real-life testing to verify performance. The creator also uses a circuit simulator to ensure accuracy and discusses the amplifier's wide bandwidth and modulation capabilities. The video concludes with a thermal performance test and insights on improving efficiency.
Takeaways
- 🔬 The video focuses on designing, simulating, and building a Class A single-transistor wideband common emitter amplifier.
- 🎯 The design goal is to deliver half a watt into a 50-ohm load, using a BD135 medium power NPN transistor for amplification.
- 🔧 The amplifier configuration includes a resistor network for bias point setting, an emitter resistor for stability and negative feedback, and an inductor in the collector for efficiency.
- 🔌 The supply voltage is calculated to be 10 volts, considering the load, collector-emitter voltage drop, and emitter resistor voltage drop.
- 🔢 The static operating current is determined to be 141.5 milliamps, which is half of the peak current.
- ⚙️ The emitter resistor is calculated to be 3.3 ohms, and the biasing resistors are calculated to be 620 ohms and 91 ohms.
- 🌡️ The amplifier is expected to have a power dissipation of almost 1.5 watts, necessitating a heatsink for the transistor.
- 📊 Simulations show a voltage gain of about 13, an efficiency of 28.6%, and a bandwidth from 10 kHz to 10 MHz.
- 🔍 Real-life testing confirms the amplifier's performance, with a wide bandwidth and good modulation following for various types of signals.
- 🌡️ Thermal performance testing indicates the importance of considering heat dissipation not only for the transistor but also for other components.
Q & A
What is the main goal of designing the class A amplifier discussed in the script?
-The main goal is to design a single transistor wideband common emitter amplifier that can deliver half a watt into a 50-ohm load.
Why is a BD135 transistor chosen as the main amplifying element?
-The BD135 is chosen because it is a medium power NPN transistor with a high enough transition frequency to allow the amplifier to work up to a few megahertz.
What is the expected theoretical maximum efficiency of the amplifier design?
-The expected theoretical maximum efficiency of the amplifier design is 50%.
How is the output voltage calculated for the amplifier?
-The output voltage is calculated based on the output load value and the output power, requiring a 5-volt RMS voltage, which translates to a 14.14-volt peak-to-peak voltage.
What is the purpose of the emitter resistor in the amplifier circuit?
-The emitter resistor serves two purposes: it helps set the voltage gain of the circuit and provides a voltage drop that contributes to heat dissipation and circuit stability.
Why is an inductor placed in the collector of the amplifier?
-An inductor is placed in the collector to maximize efficiency by minimizing the voltage drop across the collector and emitter.
How is the static operating current determined for the amplifier?
-The static operating current is determined based on the peak-to-peak load voltage and the load value, which gives a peak current of 283 milliamps, and the static current is half of this value.
What is the significance of the biasing resistors in the circuit?
-The biasing resistors are used to set the operating point of the transistor and ensure stability by providing the right amount of base current.
How is the power dissipation calculated for the transistor?
-The power dissipation is calculated by considering the static power consumption of the amplifier, which is assumed to be mostly dissipated on the transistor, resulting in almost one and a half watts.
What is the expected efficiency of the amplifier based on simulations?
-The expected efficiency based on simulations is about 35%, which is derived from the output power and the input power calculations.
How is the bandwidth of the amplifier tested and what are the results?
-The bandwidth is tested using an AC simulation, and the results show a flat response from around 10 kHz up to around 10 MHz, indicating a wide bandwidth amplifier.
Outlines
🔬 Designing a Class A Amplifier
The speaker introduces the project of designing, simulating, and building a single-transistor wideband common emitter amplifier. The goal is to deliver half a watt into a 50-ohm load using a BD135 transistor. The design includes a common emitter configuration with a resistor network for biasing, an emitter resistor for stability and negative feedback, and an inductor in the collector for efficiency. The speaker outlines the calculations for determining the required supply voltage, static operating current, and resistor values, aiming for a theoretical efficiency of 50%. The design omits impedance matching to achieve a wide bandwidth frequency response.
🧮 Calculating Component Values
The speaker delves into the calculations for the amplifier's components. They determine the necessary output voltage and supply voltage, taking into account the load, collector-emitter voltage drop, and voltage drop across the emitter resistor. The static operating current is calculated based on the peak load voltage and load value. The emitter resistor value is derived from the desired voltage drop and peak current, with a slight adjustment to a standard value. The biasing resistors are calculated based on the voltage drops and currents, with the speaker opting for standard resistor values. Power dissipation is also considered, leading to the recommendation of a heatsink for the transistor.
📡 Testing the Amplifier Circuit
The speaker tests the designed amplifier circuit using a circuit simulator. They verify the static operating point and compare the simulated collector current with the calculated values. The simulator is also used to test the circuit at different temperatures to mimic real-world operating conditions. The speaker checks the output signal, efficiency, voltage gain, and harmonics using the simulator. They find that the efficiency is slightly lower than expected, and the voltage gain is less than calculated due to the transistor's internal resistance. An AC simulation confirms a wide bandwidth from 10 kHz to 10 MHz.
🔍 Real-life Testing and Observations
The speaker assembles the amplifier circuit and tests it in real life. They measure the current consumption, which increases as the transistor warms up, and observe the output signal's amplitude and distortion at different input levels. The frequency response is tested, showing a wide bandwidth from 10 kHz to 5 MHz. The speaker performs modulation tests, including amplitude, phase, and frequency shift keying, demonstrating the amplifier's ability to handle various types of signals. Thermal performance is also evaluated, with the transistor and some resistors heating up during operation.
🔋 Efficiency and Future Considerations
The speaker discusses the efficiency of the Class A amplifier, noting that it is limited by non-ideal elements such as the collector-emitter voltage drop and the emitter resistor voltage drop. They suggest that using higher supply voltages with correct impedance matching could improve efficiency. The speaker highlights the high linearity and simplicity of the Class A amplifier, making it suitable for various applications, especially in audio and radio frequency signal amplification. They conclude by hinting at potential improvements or alternatives to the Class A design.
Mindmap
Keywords
💡Class A Amplifier
💡Common Emitter Configuration
💡Bias Point
💡Emitter Resistor
💡Collector Inductor
💡DC Isolation Capacitors
💡Supply Voltage
💡Static Operating Current
💡Voltage Gain
💡Efficiency
Highlights
Introduction to designing a single transistor wideband common emitter amplifier
Design goal to deliver half a watt into a 50 ohm load
Selection of BD135 medium power NPN transistor for its high transition frequency
Schematic overview including bias point setting and gain stabilization
Calculation of required 5 volt RMS output voltage for the amplifier
Determination of supply voltage considering collector-emitter voltage drop and emitter resistor voltage
Calculation of static operating current and its significance in amplifier design
Emitter resistor calculation based on set voltage drop and peak current
Biasing resistors calculation to ensure stability and desired current flow
Power dissipation estimation and the need for a heatsink
Expected efficiency calculation based on output and input power
Circuit simulation to verify calculations and operating point
Testing the amplifier's performance in real life with a compact design
Observation of current consumption and its relation to transistor temperature
Analysis of signal distortion at different input signal amplitudes
Frequency response test showing a wide bandwidth up to 20 megahertz
Modulation performance test including amplitude, phase, and ASK
Thermal performance measurement and the importance of considering all components' heat dissipation
Discussion on improving efficiency through higher supply voltages and impedance matching
Conclusion on the amplifier's high linearity and simplicity, suitable for various applications
Transcripts
hello
and welcome back
today i want to continue talking about
the class a amplifier by designing
simulating and building a single
transistor wideband common emitter
amplifier i mean how hard can it be
anyway
afterwards of course i will be testing
the circuit out to verify its actual
performance
like how linearly does it follow an
input signal and just how hot does it
get
and if you're curious then keep watching
[Music]
so first things first what is the design
goal
well as with all the linear amplifiers
that i will be building in this mini
series i want to deliver half a watt
into a 50 ohm load
since as a test load i will be using a
termination resistor
and if i'm delivering more power i'm
afraid i might break it
and as main amplifying element i will be
using a bd135
medium power npn transistor that should
have a high enough transition frequency
to allow the amplifier to work up to a
few megahertz
so let's start calculating the
components
now the general schematic will look
something like this
so i will be using a common emitter
amplifier configuration with a resistor
network to set the bias point and an
emitter resistor to stabilize the gain
and provide negative feedback
finally an inductor will be placed in
the collector to maximize efficiency so
this general topology should allow a
maximum theoretical efficiency of 50
and finally we will have input and
output
dc isolation capacitors
to isolate the input signal source and
the output load
and a supply voltage now i won't be
using any impedance matching
first of all to keep things simple but
also to get the wide bandwidth frequency
response
so to start calculating the components
let's start with what we actually know
so knowing the output load value and the
output power we can work out the output
voltage that we need
so we will need the 5 volt rms voltage
which translates to a 14 point 14 volt
peak to peak voltage now even though
this configuration
can supply double the supply voltage on
the output we will need a bit more than
7 volts to supply the circuit so the
supply voltage will have to be half of
the output load peak to peak voltage but
we also need to account for the minimum
collector emitter voltage drop and the
voltage that's dropping on the emitter
resistor
at peak current so to work out the
actual supply voltage we will need 7
volts for the load for the collector
emitter voltage we can take a value of 2
volts since the data sheet does provide
dc current gains at this specific value
so we'll choose this and for the emitter
resistor well this component has two
purposes
on the one side based on this component
and the load value we can set the
voltage gain of the circuit but secondly
the larger the voltage drop that occurs
on this resistor well the more heat gets
dissipated but also the exact base
emitter voltage which is temperature
dependent becomes less important in
analyzing the stability of the circuit
so by having large emitter resistor
voltages we can make the circuit more
stable
so we'll set this voltage drop to 1 volt
so the total supply voltage will end up
being 10 volts next we need to calculate
the static operating current so the
point around which the current will
oscillate as the amplifier is amplifying
and well this needs to be half of the
peak current
so we need to work out that thing first
so this can be determined based on the
peak to peak load voltage and well the
load value and this gives us a peak
current of 283 milliamps
so knowing this
we can work out the static current which
is half of this so
141.5 milliamps
now we can start working on the
components of the circuit so first
let's start off with the emitter
resistor
and we can calculate this based on the
voltage drop that we've set of 1 volt
and the peak current of 283 so this
gives us a value of 3.53 ohms
now
this isn't really a standard value so
i'll be using a resistor of 3.3 ohm
since i actually have this component
based on this value and the load we can
now work out the
expected voltage gain so based on the
load value and the emitter resistor this
gives us a voltage gain of 15
but in reality we'll be getting slightly
lower gains because of the equivalent
resistor present in the emitter of the
transistor so we don't just have the
emitter resistor on the outside we also
have a small resistor inside of the
transistor finally we can start working
on the biasing resistors
so first let's work out the exact
voltage drops on each of these so first
the voltage drop on r1 this will be
equal to the voltage dropping on the
emitter resistor plus the voltage
dropping on the base emitter junction
so we can take a base emitter voltage of
0.7 volts
and for the emitter resistor we can work
out the voltage drop based on the static
current so we're getting a total voltage
drop on r1 of 1.16 volts
now the voltage drop on r2
is well the supply of voltage minus
whatever is dropping on r1 so we get
8.84
next we need to work on the currents
and the first thing too calculate is the
base current of the main transistor
so this is calculated based on the
collector current and the gain factor so
for the exact transistor that i'll be
using i have a minimum gain
of about 100 and the static collector
current is 141 milliamps so this gives
us a base current of 1.41 milliamps now
to ensure stability in the circuit
common way of calculating the two
resistors is to set the current going
through r2 that is 10 times the base
current
and well the current going through r1 is
nine times the base current so one times
goes through the base so by this logic
we can work out that r2 will have a
current of 14.1 milliamps
and the current through r1 will be 12.7
now we know the currents we know the
voltages so we can work out the actual
resistor values
and again we're getting non-standard
values
so i'll be using a 620 ohm resistor and
a 91 ohm resistor for the two
bias setting resistors
final thing we can do is calculate the
power dissipation so since the amplifier
will consume roughly the same amount of
power whether it's doing something or
not we should calculate how much power
is being used in a static point and we
can assume that most of this power is
dissipated on the transistor so we're
getting almost one and a half watts so
definitely we should have a heatsink to
make sure that the transistor doesn't
self-destruct final thing to do we can
work out the expected efficiency
so we know the output power of half watt
we know the input power of 1.41 watts
and this gives us an efficiency of about
35 percent
so it's not 50 but it's not that bad
either so finally just to make sure that
no errors were made during these
calculations
let's check the circuit in a circuit
simulator
so here is the circuit that we've just
calculated
now i added in the various resistor
values that we've worked on
and
as transistor model
i used one that i found online
so i found this website that's somehow
related to philips
philips being one of the manufacturers
that used to produce this transistor in
the past and the nice thing about this
website is that they are providing a
model for the bd155-16
so this is a specific gain group and
this is the transistor from which i have
the practical one so i'll be using this
model and i'll be leaving a link to this
in the description so if you want to
check this website out so now back to
the circuit first thing to check is the
static operating point that's why
there's no input or output signal here
if we run the circuit we can check the
current running through the collector
which is around 130 milliamps slightly
lower than what we've calculated and
other than this current the total
current passing through the circuit is
about 144 milliamps now regarding the
current consumption it's important to
point out that the current simulation is
running at 25 degrees celsius but we can
also simulate at another temperature say
60
because the class a amplifier will heat
up in its operation so we can expect the
transistor to be a bit hotter
so if we now look at the collector
current we can see that the 130
milliamps is
the behavior at 25 degrees
but there's also a nice blue line up
here at around 142 milliamps which is at
60 degrees
so the hotter the transistor will be
running
the higher the collector current now
other than waiting for the transistor to
heat up
another thing that we could do is
increase the value of
r1 but for the moment i'll leave things
as they are
so the higher the current at which the
transistor is running
the lower the gain it will have so
that's why i'm leaving the circuit as it
is
but now i'll leave the temperature at 60
and if we now add in all of the other
components so other than the main
amplifier an input signal
coupling capacitors both on the input
and output load and
the inductor
now without going into too many details
about these other components regarding
the collector inductor
more is better in general so the more
inductance this inductor has the lower
the frequencies that the amplifier will
be able to handle
now on the other hand a very large
inductor will also have a lot of
parallel capacitance so there's a
balance that needs to be found here
similar story with the coupling
capacitors larger is better the larger
the capacitor the lower the bandwidth is
pushed to low frequencies
so anyway
if we run the circuit
we can look at the voltage present in
the collector
we can see that for the specific signal
that i'm inputting it's dropping to
almost three volts and the voltage on
the emitter resistor
is rising up to about 925 so we're
getting about two point something volts
drop on the collector emitter of the
transistor
we can also look at the signal on the
output so it's almost
14 volts peak to peak
and this is giving us an output power of
about 450 milliwatts so slightly lower
than 500 but it will do
and we can also perform some automated
measurements on the circuit so first of
all i prepared this set of measurements
to measure the efficiency and we can
look at that in the error log
so we can see that for this
configuration
we're getting about 28.6
so it's not great but it's more than 25.
now i also prepared this other set of
measurements
so this is to measure the voltage gain
of the circuit
by measuring the ratio between output
and input peak to peak voltages
and again this is present in the error
log and we're getting a value of about
13.
so it's less than the 15 that we would
be getting just based on the resistors
because we also have an equivalent
resistance in the emitter of the
transistor
finally we can look at the output signal
in an fft spectrum to see just how clean
it is
and we can see that we are getting some
extra harmonics
but the difference between the first two
peaks is around 34
35 decibels so we can call it decently
clean
now other than the transient simulation
we can also perform an ac simulation
so just to quickly see the bandwidth of
the circuit and if we look at the output
we can see we are getting a fairly flat
response
starting from around 10 kilohertz up to
around 10 megahertz so over three
decades of frequency
so this will be quite a decent wide
bandwidth amplifier
so now that we've seen that it works in
the simulator it's time to test it out
in real life
now without going into too many details
this is a really simple design after all
here is the finished circuit it's quite
compact and other than the large
heatsink for the power transistor and
the large inductor most components are
quite small
i did however use through-hole
components for the resistors to better
dissipate the heat but everything else
is surface mounted
also other than the components already
discussed
the circuit has a set of supply
decoupling capacitors and filters to
reduce any noise that would be otherwise
going out onto the supply lines
and that being said it's time to test
things out
so here is the first set of that i've
prepared
i've got my circuit is being supplied
from my power supply back here and i'm
passing the current through an ammeter
to get a more precise current
measurement
my signal generator will be providing
the signal into the circuit and the
output is connected to the
oscilloscope's first channel where i
have my 50 ohm termination resistor
so
first thing to check out
is the current consumption of the
circuit so this should be our very first
indicator to see whether the circuit is
working as expected or not so when i
connect
the power supply
you are getting about 139 point
something milliamps
and we can see that this current is
increasing
so as the transistor is warming up we
are getting more and more current
consumption
so this is very close to the total
current consumption that the simulator
was telling us about
now at the moment there is no output
signal we can see it's flat and if we
turn on the signal generator
so we're at 140 546 milliamps
with an output signal
not much has really changed so we are
still in the same region we see that
current is slowly decreasing
so again as some of the power gets
delivered transistor is getting slightly
colder so we have a small shift in the
current consumption
now if we look at the actual signal we
can see
we are getting a pretty nice sine wave
we can play around with the amplitude a
bit so if we increase the amplitude we
can see a bit of distortion occurring on
the bottom side also on the top side so
if the signal is small enough then it's
nice and symmetrical but if you increase
it too much then you start to get some
problems so we leave it like this
so at the moment we're getting 4.67
volts rms onto the 50 ohm load and this
is giving us a power of
436 milliwatts
now the first observation to make about
this is that if you want to make an
amplifier for half watt
design it for a bit more
otherwise you might run into distortions
and other problems
now anyway the current signal that we
are looking at is running at two
megahertz
so the other thing that we can play
around is the input signal frequency
so we can go up to three four five it's
starting to decrease so as frequency
increases
we start to get a smaller signal so this
is at five megahertz but we can also go
the other way
so one megahertz
let's just zoom in a bit more
for 400 kilohertz 200 100
we can see that we are still getting
roughly the same amplitude and we can go
even lower
so maybe you can already hear it it's at
10 kilohertz right now and even lower
and now the amplitude drops a lot more
so you can see that we are getting a
very wide bandwidth
but the best way to test it is not like
this
but rather with the body analysis so for
this measurement
i'm using a signal amplitude of about
900 millivolts and i'm running a sweep
between three kilohertz and 50 megahertz
just so you can see a nice and wide
response
and if we look at the measurement
so we have 10 points per decade we can
see we are getting a very nice very flat
response
all the way up to almost 20 megahertz so
between this 10 kilohertz point we're
getting an amplitude of 22 decibels
it stays relatively flat 22 22 22
all the way
up to 15.85
megahertz
and then it slowly drops off so it did
manage to make a very nice very wide
band amplifier
next we can look at modulation
performance so i left and yellow the
input signal in blue the output i'm
running at 500 kilohertz and the 500
millivolt input signal so there's no
point in checking out frequency
modulation we already saw that the
circuit has a very wide bandwidth so
let's look at some other types of
modulation first off being amplitude
modulation
so we can see that the input signal and
the output signal are almost identical
in shape maybe there's a bit of
distortion here on the bottom of the
blue signal
but that of course can be
adjusted by the input signal amplitude
otherwise the two signals are quite
identical
next we can try out phase modulation
so this is the type of modulation in
which the phase of the signal suddenly
changes
and we can see it on the input signal so
we've went from one phase to
completely different phase frequency
stayed the same but we had a very sudden
change
and we can see the same thing on the
output so the output is inverted but we
can see that the output signal very
closely followed the input signal so
there was no lag or nothing
the output followed the input exactly
final thing to look at is
ask type of modulation amplitude skip
keying and here again we can see that
the input signal and the output are very
well matched so there's no delay in the
output signal it's following the input
signal identically there's no variation
in amplitude it starts off
directly at full amplitude and then
turns off right as the input signal
disappears
so we are getting an all-around good
performance with any sort of signal
modulation
now final thing to do is test thermal
performance
so right now there is a signal passing
through the circuit
and i'm measuring the temperature on the
transistor on the screw that's holding
the transistor in place with a
thermocouple so which are getting about
51 52 degrees celsius we can also look
at the circuit with the thermal camera
so we can see the main
hot element is the transistor and the
heatsink but there's also a couple of
resistors in there which are also
heating up so when designing such an
amplifier it's also important to take
into consideration that
you're dissipating power not just on the
main amplifying element so you need to
take care with also the other components
now we can also look at what happens
with the amplifier if we turn off the
signal
so right now the signal is off
current consumption
stayed roughly the same
but since there's no more power being
delivered to the load we should be
seeing an increase in temperature since
now all of the consumed power
is dissipated on the transistor so as
with any thermal behavior
it takes the time it takes
but we can see that the temperature is
now slowly increasing
so we're at about 54 degrees and it will
probably increase a bit more
now the circuit works it's performing as
expected
but there are some interesting things to
point out regarding what could have been
done differently
specifically regarding the efficiency so
the main non-ideal elements which are
pushing us away from the
maximum theoretical efficiency
are the minimum collector emitter
voltage
not being zero but rather a couple of
volts
and also the voltage drop on the emitter
resistor
now you can use better transistors and
these voltages will drop slightly
but the fact remains that you will still
have 2-3 volts out of the supply voltage
being lost because of these non-ideal
behaviors and the need for feedback
now if you lose 3 volts out of 10 volts
it's a 30 percent loss but if you lose 3
volts out of say a hundred it's only
three percent
what i'm trying to say is that these
voltages are relatively fixed and
independent of the supply voltage
so as long as the correct impedance
matching is used on the output side you
can greatly improve efficiency
simply by using higher supply voltages
in the end the class a amplifier
provides the benefit of high linearity
which is highly priced in audio
applications and also allows the
possibility to amplify any sort of
modulation specifically useful when
talking about radio frequency signals
but it also has the important benefit of
simplicity
it can be built around a single
amplifying element
if the efficiency is not of big concern
this type of amplifier is extremely
useful and important and has a multitude
of applications
now if only there was a way to do
something about that efficiency
what's the next letter after a
anyway that will be it for today
so hope you got some instant information
to this leave your thoughts in the
comments thank you for watching make
sure to subscribe to be up to date at
tomatoes videos and see you next time
bye bye
you
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