BJT Common Collector Amplifier

Overview

The BJT common-collector amplifier is one of the three basic single-stage BJT amplifier topologies. The common collector amplifier topology is also known as a emitter follower amplifier or voltage follower. It has a high input-impedance, a low output-impedance, and has a non-inverting gain of around 1. It does NOT produce voltage gain, but is able to provide current gain, and therefore overall power gain. Because of it’s “buffering” capabilities it is often used between high-impedance inputs and low-impedance outputs (i.e. it is good for driving power-hungry loads)¹.

The MOSFET analogue to the BJT common collector amplifier is the common drain amplifier.

Properties:

Voltage Gain	Low
Current Gain	High
Power Gain	Medium
Input Impedance	High
Output Impedance	Low
Phase Shift	0°

Basic Common Collector Amplifier

A schematic of a basic common-collector BJT amplifier is shown below:

The output voltage is almost equal to the input voltage, except for an approximately \(0.7V\) diode drop across the base-emitter junction. This means that the amplifier has a voltage gain of almost unity (1), or \(0dB\).

$$\begin{align} v_{out} = v_{in} - 0.7V \\ \end{align}$$

Here is a graph of \(v_{in}\) vs. \(v_{out}\) for the above circuit, with \(R1=1k\Omega\):

(Micro-Cap simulation file: circuit.cir)

The common-collector amplifier is simulated in circuitjs below:

Common Collector Amplifier With AC Coupling And DC Bias

A more useful common-collector amplifier can be made which AC couples the input and provides a DC bias point at the base of the BJT. Below is a schematic of one built with a NPN transistor:

The small-signal AC model for this circuit is shown below. DC voltage rails and capacitors are shorted.

\(r_e\) is the small-signal emitter resistance which is internal to the BJT.

Unloaded Voltage Gain

The unloaded small-signal voltage gain of a common-collector amplifier is found by ignoring \(R_L\) in the small-signal AC model of the common-collector circuit. By definition the gain is:

$$\begin{align} A_V = \frac{v_{out}}{v_{in}} \\ \end{align}$$

Remember that \(v_{in}\) and \(v_{out}\) are lower case and represent changes in the signal (i.e. deltas, and ignore their DC levels). So a change in \(v_{out}\) is just a change of emitter voltage, and a change in \(v_{in}\) is just a change in base voltage. We can also apply Ohm’s law to get:

$$\begin{align} A_V &= \frac{v_e}{v_b} \nonumber \\ &= \frac{i_c R_E}{i_c(r_e + R_E)} \nonumber \\ &= \frac{R_E}{r_e + R_E} \\ \end{align}$$

Input Impedance

The input impedance looking into the base of the transistor is:

$$\begin{align} Z_{in(base)} &= \beta (r_e + R_E) \nonumber \\ \end{align}$$

Then the total input impedance is the base input impedance in parallel with both base resistors:

$$\begin{align} Z_{in} &= Z_{in(base)} || R_{B1} || R_{B2} \nonumber \\ \end{align}$$

Further Reading

Common-Collector Amplifier Design by Kenneth A. Kuhn goes into great detail about the amplifier design with precise equations and considerations². It’s much better than what I have here!

References