16- Common Collector Amplifier

Common Collector Amplifier

Common Collector Amplifiers produce an output voltage across its emitter load which is in-phase with the input signal

The Common Collector Amplifier is another type of bipolar junction transistor, (BJT) configuration where the input signal is applied to the base terminal and the output signal taken from the emitter terminal. Thus the collector terminal is common to both the input and output circuits. This type of configuration is called Common Collector, (CC) because the collector terminal is effectively “grounded” or “earthed” through the power supply.

In many ways the common collector configuration (CC) is the reverse of the common emitter (CE) configuration as the connected load resistor is changed from the collector terminal for RC to the emitter terminal for RE.

The common collector or grounded collector configuration is commonly used where a high impedance input source needs to be connected to a low impedance output load requiring a high current gain. Consider the common collector amplifier circuit below.

Common Collector Amplifier using an NPN Transistor

Resistors R1 and R2 form a simple voltage divider network used to bias the NPN transistor into conduction. Since this voltage divider lightly loads the transistor, the base voltage, VBcan be easily calculated by using the simple voltage divider formula as shown.

Voltage Divider Network

With the collector terminal of the transistor connected directly to VCC and no collector resistance, (RC = 0) any collector current will generate a voltage drop across the emitter resistor RE.

However, in the common collector amplifier circuit, the same voltage drop, VE also represents the output voltage, VOUT.

Ideally we would want the DC voltage drop across RE to be equal to half the supply voltage, VCC to make the transistors quiescent output voltage sit somewhere in the middle of the characteristics curves allowing for a maximum unclipped output signal. Thus the choice of RE depends greatly on IB and the transistors current gain Beta, β.

As the base-emitter pn-junction is forward biased, base current flows through the junction to the emitter encouraging transistor action causing a much larger collector current, IC to flow. Thus the emitter current is a combination of base current and collector current as: IE = IB + IC. However, as the base current is extremely small compared to the collector current, the emitter current is therefore approximately equal to the collector current. Thus IE ≈ IC

As with the common emitter (CE) amplifier configuration, the input signal is applied to the transistors base terminal, and as we said previously, the amplifiers output signal is taken from the emitter emitter terminal. However, as there is only one forward biased pn-junction between the transistors base and its emitter terminal, any input signal applied to the base passes directly through the junction to the emitter. Therefore the output signal present at the emitter is in-phase with the applied input signal at the base.

As the amplifiers output signal is taken from across the emitter load this type of transistor configuration is also known as an Emitter Follower circuit as the emitter output “follows” or tracks any voltage changes to the base input signal, except that it remains about 0.7 volts (VBE) below the base voltage. Thus VIN and VOUT are in-phase producing zero phase difference between the input and output signals.

Having said that, the emitters pn-junction effectively acts as a forward biased diode and for small AC input signals this emitter diode junction has a resistance given by: r’e = 25mV/Ie where the 25mV is the thermal voltage of the junction at room temperature (25oC) and Ie is the emitter current. So as the emitter current increases, the emitter resistance decreases by a proportional amount.

The base current which flows through this internal base-emitter junction resistance also flows out and through the externally connected emitter resistor, RE. These two resistances are series connected thus acting as a potential divider network creating a voltage drop. Since the value of r’e is very small, and RE is much larger, usually in the kilohms (kΩ) range, the magnitude of the amplifiers output voltage is therefore less than its input voltage.

However, in reality the magnitude of the output voltage (peak-to-peak) is generally in the 98 to 99% value of the input voltage which is close enough in most cases to be considered as unity gain.

We can calculate the voltage gain, VA of the common collector amplifier by using the voltage divider formula as shown assuming that the base voltage, VB is actually the input voltage, VIN.

Common Collector Amplifier Voltage Gain

So the common collector amplifier cannot provide voltage amplification and another expression used to describe the common collector amplifier circuit is as a Voltage Follower Circuit for obvious reasons. Thus since the output signal closely follows the input and is in-phase with the input the common collector circuit is therefore a non-inverting unity voltage gain amplifier.

Common Collector Amplifier Example No1

A common collector amplifier is constructed using an NPN bipolar transistor and a voltage divider biasing network. If R1 = 5k6Ω, R2 = 6k8Ω and the supply voltage is 12 volts. Calculate the values of: VB, VC and VE, the emitter current IE, the internal emitter resistance r’e and the amplifiers voltage gain AV when a load resistance of 4k7Ω is used. Also draw the final circuit and corresponding characteristics curve with load line.

1. Base biasing voltage, VB

2. Collector voltage, VC. As there is no collector load resistance, the transistors collector terminal is connected directly to the DC supply rail, so VC = VCC = 12 volts.

3. Emitter biasing voltage, VE

4. Emitter Current, IE

5. AC Emitter Resistance, r’e

6. Voltage gain, AV

Common Collector Amplifier Circuit with Load Line

Common Collector Input Impedance

Although the common collector amplifier is not very good at being a voltage amplifier, because as we have seen, its small signal voltage gain is approximately equal to one (AV ≅ 1), it does however make a very good voltage buffer circuit due to its high input (ZIN) and low output (ZOUT) impedances, providing isolation between an input signal source from a load impedance load.

Another useful feature of the common collector amplifier is that it provides current gain (Ai) as long as it is conducting. That is it can pass a large current flowing from the collector to the emitter, in response to a small change to its base current, IB. Remember that this DC current only sees RE as there is no RC. Then the DC current is simply: VCC/RE which can be large if RE is small.

Consider the basic common collector amplifier or emitter follower configuration below.

Common Collector Amplifier Configuration

For AC analysis of the circuit, the capacitors are shorted and VCC is shorted (zero impedance). Thus the equivalent circuit is given as shown with the biasing currents and voltages given as:

The Input Impedance, ZIN of the common collector configuration looking into the base is given as:

But as Beta, β is generally much greater than 1 (usually above 100), the expression of: β + 1 can be reduced to just Beta, β as multiplication by 100 is virtually the same as multiplying by 101. Thus:

Common Collector Amplifier Base Impedance

Where: β is the transistors current gain, Re is the equivalent emitter resistance, and r’e is the ac resistance of the emitter-base diode. Note that since the combined value of Re is generally much greater than the diodes equivalent resistance, r’e (kilo-ohms compared to a few ohms) the transistors base impedance can be given as simply: β*Re.

An interesting point to notice here is that the the transistors input base impedance, ZIN(base) can be controlled by the value of either the emitter leg resistor, RE or the load resistor RL as they are parallel connected.

While the equation above gives us the input impedance looking into the base of the transistor, it does not give us the true input impedance that the source signal would see looking into the complete amplifier circuit. For that we need to consider the two resistors which make up the voltage divider biasing network. Thus:

Common Collector Amplifier Input Impedance

Common Collector Example No2

Using the previous common collector amplifier circuit above, calculate the input impedances of the transistors base and amplifier stage if the load resistance, RL is 10kΩ and the NPN transistors current gain is 100.

1. AC Emitter Resistance, r’e

2. Equivalent Load Resistance, Re

3. Transistors Base Impedance, ZBASE

2. Amplifier Input Impedance, ZIN(STAGE)

As the transistors base impedance of 322kΩ is much higher than the amplifiers input impedance of only 2.8kΩ, thus the input impedance of the common collector amplifier is determined by the ratio of the two biasing resistors, R1 and R2.

Common Collector Output Impedance

To determine the CC amplifiers output impedance ZOUT looking from the load back into the amplifiers emitter terminal, we must first remove the load as we want to see the effective resistance of the amplifier that is driving the load. Thus the AC equivalent circuit looking into the amplifiers output is given as:

From above, the input impedance of the base circuit is given as:RB = R1||R2. The current gain of the transistor is given as: β. Thus the output equation is given as:

We can see then that the emitter resistor, RE is effectively in parallel with the whole impedance of the transistor looking back into its emitter terminal.

If we calculate the output impedance of our common emitter amplifier circuit using the component values from above, it would give an output impedance ZOUT of less than 50Ω (49.5Ω) which is much smaller than the higher input impedance, ZIN(BASE) calculated previously.

Thus we can see then that the Common Collector Amplifier configuration has, from calculation, a very high input impedance and a very low output impedance allowing it to drive a low impedance load. In fact due to the CC amplifiers relatively high input impedance and very low output impedance it is commonly used as a unity gain buffer amplifier.

Having determined that the output impedance, ZOUT of our example amplifier above is approximately 50Ω by calculation, if we now connect the 10kΩ load resistor back into the circuit, the resulting output impedance will be:

Although the load resistance is 10kΩ, the equivalent output resistance is still low at 49.3Ω. This is because RL is large compared with ZOUT, thus for maximum power transfer, RL must equal ZOUT. As the voltage gain of the common collector amplifier is considered to be unity (1), the amplifiers power gain must be equal to its current gain, as P = V*I.

Since the common collector current gain is defined as the ratio of the emitter current to the base current, γ = IE/IB = β + 1, it therefore follows that the amplifiers current gain must be approximately equal to Beta (β) as β + 1 is virtually the same as Beta.

Common Collector Summary

We have seen in this tutorial about the Common Collector Amplifier that it gets its name because the collector terminal of the BJT is common to both the input and output circuits as there is no collector resistance, RC.

The voltage gain of the common collector amplifier is approximately equal to unity (Av ≅ 1) and that its current gain, Ai is approximately equal to Beta, (Ai≅β) which depending on the value of the particular transistors Beta value can be quiet high.

We have also seen through calculation, that the input impedance, ZIN is high while its output impedance, ZOUT is low making it useful for impedance matching (or resistance-matching) purposes or as a buffer circuit between a voltage source and a low impedance load.

As the the common collector (CC) amplifier receives its input signal to the base with the output voltage taken from across the emitter load, the input and output voltages are “in-phase” (0o phase difference) thus the common collector configuration goes by the secondary name of Emitter Follower as the output voltage (emitter voltage) follows the input base voltage.