BJT: Collector Feedback Bias Explained

Summary

This video from the All About Electronics YouTube channel explores the Collector Feedback Biasing configuration of a Bipolar Junction Transistor (BJT). It explains how this configuration uses negative feedback from the collector to the base through resistor Rb to stabilize the operating point against external parameter changes like temperature. The script delves into DC analysis, deriving expressions for base current, collector current, and voltage Vce. It also discusses the impact of β variations on the operating point and suggests that for utmost stability, a voltage divider configuration may be preferable. The video concludes with an invitation for viewers to engage with the content through comments and subscriptions.

Takeaways

• 📚 The video discusses the Collector Feedback Biasing configuration of a Bipolar Junction Transistor (BJT).
• 🔄 The purpose of this biasing configuration is to stabilize the operating point of the BJT through negative feedback from the collector to the base terminal via resistor Rb.
• 🌡️ The circuit is designed to minimize changes in the operating point due to external parameters like temperature.
• 🔌 For DC analysis, capacitors in the circuit are considered as open circuits.
• 🛠️ The DC analysis involves marking currents and applying Kirchhoff's Voltage Law (KVL) to find expressions for base current (Ib), collector current (Ic), and voltage Vce.
• ⚖️ The collector current Ic is approximately equal to the current through resistor Rc, and can be considered much larger than the base current Ib when β is high.
• 🔢 The base current Ib can be calculated using the formula: Ib = (Vcc - Vbe) / (Rb + β*Rc).
• 🔗 The collector current Ic is directly proportional to the base current Ib, with the relationship Ic = β*Ib.
• 📉 The voltage Vce across the collector and emitter can be found using the formula: Vce = Vcc - Ic*Rc.
• 🔄 An improved stability can be achieved by adding an emitter resistor, creating a collector and emitter feedback bias configuration.
• 🔍 The operating point's sensitivity to changes in β is demonstrated through an example, showing variations in Ic and Vce with different β values.
• 📉 The example illustrates that even with collector feedback, the operating point can still vary significantly with changes in β.
• ⚠️ The condition for the base current Ib to be independent of β is when Rb is much less than β*(Rc + Re), which is not always practical for BJT operation.
• 📉 The video concludes that while collector feedback biasing offers some improvement in stability, it is not as stable as a voltage divider configuration for very stable operating points.

Q & A

What is the main purpose of the collector feedback biasing configuration in a BJT?

The main purpose of the collector feedback biasing configuration in a BJT is to stabilize the operating point through negative feedback, minimizing changes in the operating point due to external parameter variations such as temperature.

How does the collector feedback biasing configuration provide negative feedback in a BJT?

The collector feedback biasing configuration provides negative feedback by connecting a resistor (Rb) from the collector to the base terminal, which helps to stabilize the operating point by reducing the base current if the collector current increases, due to temperature or other factors.

What is the significance of the resistor Rc in the collector feedback biasing configuration?

The resistor Rc is significant in the collector feedback biasing configuration as it creates a voltage drop when the collector current (Ic) flows through it. This voltage drop affects the collector voltage (Vc), which in turn influences the base current (Ib) and helps in stabilizing the operating point.

How does an increase in temperature affect the collector current Ic and the base current Ib in the collector feedback biasing configuration?

An increase in temperature can cause the collector current Ic to increase, which leads to an increased voltage drop across the resistor Rc, reducing the collector voltage (Vc). As a result, the base current Ib will decrease, which in turn reduces the collector current Ic, due to the relationship Ic = β * Ib.

What is the role of the KVL (Kirchhoff's Voltage Law) in the DC analysis of the collector feedback biasing configuration?

The KVL is used in the DC analysis to derive the expressions for the base current (Ib), collector current (Ic), and the voltage across the collector-emitter (Vce). It helps in understanding how the voltages and currents are related in the circuit and how they contribute to the stabilization of the operating point.

What is the expression for the base current Ib in the collector feedback biasing configuration?

The expression for the base current Ib in the collector feedback biasing configuration is Ib = (Vcc - Vbe) / (Rb + β * Rc), where Vcc is the supply voltage, Vbe is the base-emitter voltage, Rb is the base resistor, β is the current gain, and Rc is the collector resistor.

How does the addition of an emitter resistor improve the stability of the collector feedback biasing configuration?

The addition of an emitter resistor (Re) provides additional negative feedback to the circuit, which helps to further stabilize the operating point. The emitter resistor creates a voltage drop that affects the base current, enhancing the circuit's ability to maintain a stable operating point despite variations in external parameters.

What is the condition for the base current Ib to become independent of the value of β in the collector feedback biasing configuration?

The condition for the base current Ib to become independent of the value of β is when Rb is much less than β * (Rc + Re). In this case, the base current Ib can be approximated as (Vcc - Vbe) / β * (Rc + Re), and the collector current Ic becomes independent of β variations.

What are the limitations of the collector feedback biasing configuration in terms of stability?

While the collector feedback biasing configuration provides improved stability over fixed bias configurations, it still experiences variations in the operating point due to changes in external parameters. The stability is not as high as that provided by a voltage divider configuration, which is why it is widely used for applications requiring a very stable operating point.

How does the change in the value of β affect the operating point of the BJT in the collector feedback biasing configuration?

A change in the value of β affects the operating point by altering the base current Ib and consequently the collector current Ic. An increase in β can lead to an increase in Ic and a decrease in Vce, as demonstrated in the example provided in the script. However, the impact of β variations can be mitigated by carefully selecting resistor values.

Outlines

00:00

🔬 Introduction to Collector Feedback Biasing in BJTs

The video begins with an introduction to the Collector Feedback Biasing configuration for Bipolar Junction Transistors (BJTs). The speaker explains the importance of a stable operating point in BJTs, which should remain consistent even when external parameters like temperature change. The Collector Feedback Biasing configuration aims to achieve this stability through negative feedback from the collector to the base terminal via resistor Rb. The DC analysis is introduced, and capacitors are considered as open circuits in this context. The video then intuitively explains how an increase in collector current due to temperature would lead to a decrease in collector voltage, which in turn reduces the base current and collector current, thus stabilizing the operating point. The DC analysis continues with marking currents and deriving expressions for base current and collector current, highlighting the role of resistors Rb and Rc in this process.

05:01

🔍 DC Analysis and Stability Improvement with Emitter Resistor

This paragraph delves deeper into the DC analysis of the Collector Feedback Biasing configuration, providing mathematical expressions for base current, collector current, and voltage Vce. The video explains that although the circuit attempts to stabilize the operating point through negative feedback, it lacks sufficient stability. To address this, an additional emitter resistor can be introduced, leading to a configuration known as Collector and Emitter Feedback Bias. The current markings are reiterated, and the Kirchhoff Voltage Law (KVL) is applied to derive new expressions for base current and collector current, considering the emitter resistor. The paragraph concludes with expressions for the base current and Vce, noting that under certain conditions, the collector current can become independent of the β value, thus improving stability.

10:03

📚 Example Calculation and Impact of β Variation

The speaker provides a practical example to illustrate the impact of β variation on the operating point of the BJT circuit. The base current, collector current, and Vce are calculated using the derived expressions, resulting in specific values for these parameters. The video then demonstrates what happens when the β value increases, showing a recalculation of the base current, collector current, and Vce. The results indicate that an increase in β leads to an increase in collector current and a decrease in Vce, highlighting the circuit's sensitivity to β variation. The example also emphasizes the importance of the condition Rb << β*(Rc + Re) for achieving stability and the challenges of satisfying this condition in practice.

15:06

🚀 Conclusion and Recommendations for Stable Operating Points

In conclusion, the video acknowledges that while the Collector Feedback Biasing configuration offers some improvement in stability, it is still susceptible to variations in the operating point due to external factors. The speaker recommends the voltage divider configuration for those seeking a more stable operating point, as it can provide better stability with proper selection of component values. The video wraps up with an invitation for viewers to ask questions or provide suggestions in the comments and encourages viewers to like and subscribe for more educational content.

Keywords

💡Collector Feedback Biasing

Collector Feedback Biasing is a configuration used in Bipolar Junction Transistors (BJTs) to stabilize the operating point through negative feedback from the collector to the base terminal. In the context of the video, it is a method to minimize changes in the operating point due to external parameter variations like temperature. The script explains that this configuration uses resistor Rb to provide feedback, which helps in stabilizing the collector current Ic and the collector-emitter voltage Vce.

💡BJT (Bipolar Junction Transistor)

A Bipolar Junction Transistor (BJT) is a type of transistor that utilizes both electron and hole charge carriers. The video's theme revolves around different biasing configurations of BJTs. The script discusses how the collector feedback biasing configuration of a BJT is designed to maintain a stable operating point despite changes in external parameters.

💡Operating Point

The operating point of a transistor refers to the DC current and voltage conditions at which it operates in a circuit. The video emphasizes the importance of a stable operating point in BJTs, especially under varying conditions like temperature. The collector feedback biasing configuration is shown to help stabilize this operating point through negative feedback.

💡Negative Feedback

Negative feedback in electronic circuits is a process where a portion of the output signal is fed back and subtracted from the input. In the script, it is explained that the collector feedback biasing configuration uses negative feedback to stabilize the operating point of a BJT, as any change in the collector current due to external factors will be counteracted by the feedback mechanism.

💡DC Analysis

DC Analysis involves examining the Direct Current (DC) components of a circuit, ignoring the transient behavior and focusing on the steady-state conditions. The video script describes performing a DC analysis on the collector feedback biasing configuration to understand how the circuit stabilizes the operating point under steady-state conditions.

💡Resistor Rb

Resistor Rb is a component in the collector feedback biasing configuration that plays a crucial role in providing negative feedback from the collector to the base of the BJT. The script explains that this resistor is instrumental in stabilizing the operating point by adjusting the base current Ib in response to changes in the collector current Ic.

💡Base Current (Ib)

Base current (Ib) is the current flowing into the base of a BJT. The video script uses the concept of base current to explain how changes in Ib affect the collector current Ic, and subsequently, the operating point of the BJT. The base current is calculated using the DC voltages and resistor values in the circuit.

💡Collector Current (Ic)

Collector current (Ic) is the current flowing out from the collector of a BJT. The script discusses how the collector current is stabilized through the collector feedback biasing configuration, and it is shown that Ic is directly proportional to the base current (Ic = β*Ib), where β is the current gain of the BJT.

💡Voltage Divider Configuration

A voltage divider configuration is a method of biasing a BJT that provides a very stable operating point. The video script mentions this configuration as a preferred choice when a highly stable operating point is required, as it can offer better stability compared to the collector feedback biasing configuration.

💡Emitter Resistor (Re)

An emitter resistor (Re) is an additional component that can be included in the collector feedback biasing configuration to enhance stability. The script explains that by adding an emitter resistor, more negative feedback can be introduced into the circuit, which helps to further stabilize the operating point of the BJT.

Highlights

Introduction to the Collector Feedback Biasing configuration of the BJT.

Importance of stable biasing circuit design for BJTs to handle changes in external parameters like temperature.

Explanation of how the collector feedback biasing configuration uses negative feedback for stabilizing the operating point.

DC analysis approach by treating capacitors as open circuits.

Intuitive understanding of the circuit's stabilization mechanism through an example of increased collector current due to temperature.

DC analysis of the circuit, starting with marking all currents.

Assumption that the collector current is much larger than the base current for high β values.

Application of Kirchhoff's Voltage Law (KVL) to find the base current expression.

Derivation of expressions for base current, collector current, and voltage Vce.

Introduction of an improved stability configuration with an emitter resistor.

Derivation of new expressions for base current, collector current, and voltage Vce with the emitter resistor.

Condition for Rb to be negligible and its impact on the independence of Ic from β.

Practical limitations of making Rb very small for operating BJTs in the linear region.

Example calculation to illustrate the impact of β variation on the operating point.

Comparison of operating points with different β values and their effects on Ic and Vce.

Discussion on the limitations of the collector feedback biasing configuration and its comparison with other configurations.

Recommendation for preferring a voltage divider configuration for a very stable operating point.

Conclusion summarizing the slight improvement in stability provided by the collector feedback biasing configuration.

Invitation for questions, suggestions, and engagement through likes and subscriptions.

Transcripts

Hey friends, welcome to the YouTube channel ALL ABOUT ELECTRONICS.

So, in this video, we will learn about the Collector Feedback Biasing configuration of

the BJT.

Now, in the earlier videos, we had seen the different biasing configurations of the BJT.

And we had seen that for the BJT, the designed biasing circuit should be such that even if

there is a change in the external parameters, like a temperature then there should be a

minimum change in the operating point.

So, in this collector feedback biasing configuration, the circuit tries to stabilize the operating

point through the negative feedback.

And as its name suggests, here the feedback is provided from the collector to base terminal

via this resistor Rb.

Now, here as we are interested in the DC analysis, so for the DC voltages, these capacitors will

act as an open circuit.

And before we jump into the DC analysis, lets intuitively understand how this circuit tries

to stabilize the operating point.

So, let's say, due to the temperature, if the collector current Ic increases, then the

voltage drop across this resistor Rc will also increase.

And due to that, the collector voltage or the voltage at this node will reduce.

Now, if we assume this base-emitter junction is forward biased then the voltage at this

node is equal to Vbe.

Or that is equal to roughly 0.7V.

So, as the voltage Vc reduces, then the base current Ib will also reduce.

And due to that, the collector current Ic will also reduce.

Because this collector current Ic is equal to β*Ib.

So, in this way, this circuit tires to stabilize any change in the collector current.

But let's do the DC analysis, and let's understand how well the circuit is able to stabilize

the operating point.

And first of all, let's mark all the currents in the circuit.

Now, the first thing if you notice over here, the current through the resistor Rc is equal

to Ic'.

And it is the summation of the base current and the collector current.

And here, this collector current Ic is equal to β*Ib.

So, if the value of β is very high, in that case, this collector current is much larger than

the base current.

And due to that, in this expression, we can neglect the base current.

Or approximately we can say that this current Ic' is equal to Ic.

So, now we can say that the current through the resistor Rc is equal to Ic, while the

current through the resistor Rb is equal to Ib.

And the voltage between this base and the emitter terminal is equal to Vbe.

So, now to find the base current, let's apply the KVL on the input side.

So, applying the KVL, we can write, voltage Vcc, minus Ic*Rc, that is the voltage drop

across this resistor Rc, minus Ib*Rb, minus Vbe, that is the voltage drop between this

base and the emitter terminal, is equal to 0.

That means voltage Vcc- Vbe = Ib*Rb + Ic*Rc.

Now, we know that the collector current Ic is equal to β* Ib.

So, we can write this expression as voltage Vcc - Vbe = Ib*Rb + β*Ib*Rc.

Or we can say that the base current Ib is equal to voltage

Vcc - Vbe / (Rb +β*Rc) So, this will be the expression of the base

current.

And we know that the collector current Ic can be given as β*Ib.

So, once we know the value of this collector current, then by applying the KVL on this

output side, we can find the value of the voltage Vce.

So, applying the KVL we can write, voltage Vcc- Ic*Rc, that is the drop across this resistor

Rc, minus voltage Vce, that is equal to zero.

Or we can say that this voltage Vce is equal to Vcc - Ic*Rc.

And in this way, we got the expressions for the base current, the collector current, and

the voltage Vce for the given configuration.

Now, although through the negative feedback, this circuit tries to stabilize the operating

point, but here there isn't enough negative feedback.

So, to improve the stability, the additional feedback can be applied through the emitter

resistor.

So, this circuit is the collector feedback bias with the emitter resistor.

And this circuit is also known as the collector and the emitter feedback bias.

So, now let's find the expressions for the collector current and the voltage Vce.

And for that, first of all, let's mark the currents in the given circuit.

Now, once again here the current through the resistor Rc is equal to Rc'.

And this current Ic' is equal to Ic +Ib.

But as I said earlier, we can approximately say that this current Ic' is equal to Ic.

Similarly, here we are assuming that this collector current is approximately equal to

emitter current.

So, considering that now let's find the expression of the base current.

And for that, now let's apply the KVL on this input side.

So, applying the KVL we can write, voltage Vcc - Ic*Rc, that is the drop across this

resistor Rc, minus Ib*Rb, minus voltage Vbe, minus Ie*Re, that is the drop across this

resistor Re, that is equal to 0.

Now, here we are assuming that this emitter current and the collector current are almost

the same.

So, we can replace this emitter current with the collector current.

And we can write this expression as voltage Vcc - Vbe = Ib*Rb + Ic*(Rc + Re)

And here this collector current Ic can be given as β*Ib.

So, from this, we can say that, voltage Vcc - Vbe = Ib*Rb + β*Ib*(Rc +Re)

Or we can say that the base current Ib is equal to voltage Vcc - Vbe / (Rb +β*(Rc +Re))

So, this will be the expression of the base current.

And we know that this collector current Ic is equal to β*Ib.

So, in this way, we can also find the collector current.

And once we know the value of this collector current, then we can easily find this voltage

Vce.

And for that let's apply the KVL on this output side.

So, applying the KVL we can write, voltage Vcc - (Ic*Rc) - Vce, that is the drop between

these two terminals, minus Ie*Re, that is equal to 0.

Or we can say that voltage Vce is equal to Vcc - Ic*(Rc +Re).

so, this will be the expression of the voltage Vce.

So, in this way, we got the values of the base current, the collector current, and the

voltage Vce.

Now, here if you observe, the value of Rb is much less than this term, that is the β*(Rb

+ Re), in that case, this resistor Rb can be neglected.

And in that case, this base current Ib can be given as Vcc - Vbe / β*(Rc + Re)

And as the collector current Ic is equal to β*Ib, so this β will get cancel out.

And due to that, this current Ic will become independent of the value of β.

That means whenever this condition is satisfied then this collector current Ic will become

independent of the variation in the β.

But practically, to operate this BJT in the linear region, it is not possible to make

the value of Rb very small.

And it will get clear to you, once we go through the example.

And in that example, we will also see, how the operating point of the circuit changes,

if there is a variation in the β.

So, let's take one example.

So, in this example, first of all, let's find the value of the collector current and the

voltage Vce.

And for that, first of all, let's find the value of the base current using this expression.

So, we can say that the base current is equal to 12V - 0.7 V/ (300 kΩ + 50(4.7 kΩ + 1

kΩ)) And that is equal to 19.

6 uA.

So, this will be the value of the base current.

And the collector current Ic can be given as β*Ib.

That means this collector current Ic is equal to (50*19.6 uA).

And that is equal to 0.965 mA.

And once we get the value of this collector current, then using this expression we can

find the value of the voltage Vce.

So, this voltage Vce can be given as 12V - (0.965 mA)(4.7 kΩ + 1 kΩ)

And that is equal to 6.5 V.

So, in this way, the value of the collector current Ic is equal to 0.965 mA, while the

value of the voltage Vce is equal to 6.5V. And if you see over here, on the load line,

the operating point would be somewhere around here.

Because here, the maximum value of the voltage Vce could be equal to 12V, while the maximum

collector current is equal to 12V / 5.7 kΩ.

That is equal to 2.1 mA.

Now, in the same circuit, let's see what happens when the value of the β increases to 100.

So, once again let's find the value of the base current using this expression.

That means the base current Ib can be given as 12V - 0.7V / (300 kΩ + 100 (5.7 kΩ))

And if we calculate the value, then this base current Ib is equal to 12.98 uA.

And the collector current Ic is equal to β*Ib.

That means this collector current Ic is equal to (100* 12.98uA)

That is equal to 1.298 mA.

Or roughly we can say that it is equal to 1.3 mA.

And once we know the value of this collector current, then using this expression we can

find the value of the voltage Vce.

So, this voltage Vce is equal to 12V - (1.3 mA* 5.7 kΩ), which is the summation of this

4.7kΩ and 1 kΩ.

So, if we calculate the value, then this voltage Vce will come out as 4.6 V.

That means now, due to the change in the value of β, the voltage Vce has become 4.6 V, while

the value of the collector current Ic has become 1.3 mA.

That means with the increase in the value of β by 100 percent, this collector current

has increased by roughly 30 percent, while the value of the voltage Vce is roughly reduced

by 34 %. That means we can say that, although this

collector feedback provides the improved performance over the fixed bias configuration, but still

there is a variation in the operating point.

Now, if you notice over here, in this example, this condition that is Rb is much less than

β* (Rc +Re)is not satisfied.

Because over here, the value of Rb is 300 kΩ, while the value of β* (Rc +Re) is equal

to 570 kΩ, considering the value of beta is equal to 100.

And if we consider the value of beta as 50, in that case, this term will be equal to 285

kΩ.

But to satisfy this condition, suppose if we reduce the value of Rb by 10 times or increase

the value of this Rc and Re by 10 times, in that case, the operating point would go near

the saturation.

And we are no longer able to use this BJT as an amplifier.

And even you can try it by yourself.

So, try to get the operating point whenever the value of Rb is equal to 30 kΩ.

So, in conclusion, this configuration provides a slight improvement in stability, but still,

there is a variation in the operating point due to the external parameters.

And when someone requires a very stable operating point, then one should prefer a voltage divider

configuration over the other configurations.

Becuase by properly selecting the values, that configuration provides better stability.

And that is why it is the widely used biasing configuration.

So, in the next couple of videos, we will talk more about the stability.

But I hope in this video, you understood the collector feedback biasing configuration of

the BJT.

So, if you have any questions or suggestions, do let me know here in the comment section

below.

If you like this video, hit the like button and subscribe to the channel for more such

videos.