BJT Common Collector Amplifier

mbedded.ninja Author

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°

note

Lower case letters used below represent changes in quantities, e.g. $V_C$ is the voltage at the collector, whilst $v_c$ is the change in voltage at the collector, $\Delta V_C$ .

Basic Common Collector Amplifier

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

The basic schematic of a common-collector BJT amplifier.

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 $R1=1k\Omega$ :

$V_{out}$ vs. $V_{in}$ for a basic common-collector BJT amplifier.

(Micro-Cap simulation file: circuit.cir)

The common-collector amplifier is simulated in circuitjs below:

(full page version)

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:

Schematic of an AC-coupled common-collector NPN BJT amplifier.

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

Small-signal AC model for the AC-coupled common-collector BJT amplifier.

$r_e$ 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}

Overview​

Basic Common Collector Amplifier​

Common Collector Amplifier With AC Coupling And DC Bias​

Unloaded Voltage Gain​

Input Impedance​

Further Reading​

Footnotes​

Overview

Basic Common Collector Amplifier

Common Collector Amplifier With AC Coupling And DC Bias

Unloaded Voltage Gain

Input Impedance

Further Reading

Footnotes