BJT COMMON COLLECTOR AMPLIFIER

# BJT Common Collector Amplifier

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## 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°
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 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.

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}

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

1. James M. Fiore (2022, May 23). Common Collector Amplifier. LibreTexts: Engineering. Retrieved 2022-08-11, from https://eng.libretexts.org/Bookshelves/Electrical_Engineering/Electronics/Book%3A_Semiconductor_Devices_-_Theory_and_Application_(Fiore)/07%3A_BJT_Small_Signal_Amplifiers/7.4%3A_Common_Collector_Amplifier↩︎

2. Kenneth A. Kuhn. Common-Collector Amplifier Design. Retrieved 2022-09-03, from https://www.kennethkuhn.com/students/ee351/bjt_cc_amplifier_design.pdf↩︎ ↩︎

## Authors

### Geoffrey Hunter

Dude making stuff.