improve a discrete voltage buffer with two transistors: a complementary emitter follower

The Emitter Follower

The Emitter Follower or Common Collector transistor amplifier circuit consists of one transistor and one resistor. It is a particular case of the more general Voltage Follower or Voltage Buffer.

Its output voltage follows the input voltage, meaning the output is ideally the same--with the caveat of a voltage offset.* It doesn't amplify or attenuate the voltage, but its high input impedance and low output impedance act to isolate or buffer the incoming signal from downstream circuit loads.

*When using the follower with an NPN or PNP transistor, the output will be offset by a silicon junction drop of about 0.7V. The offset amount will change with temperature as the junction. In many cases this has little or no impact on the application.

To drastically reduce the offset and the temperature variation, add one transistor.

The Dual Complementary Emitter Follower

The solution is to introduce another circuit with the same temperature dependence but moving in the opposite direction. They will tend to cancel each other out. Also, if the circuit could add the opposite offset voltage, the buffered voltage would be back where it started; a Voltage Follower. Here's a way to do it.

The input resistor is not needed in theory, but is useful in practice to provide some protection from spike energy and to current limit under fault conditions, with no negative effects.

Start with the standard emitter follower. Then add a 2nd follower, but using a PNP transistor. This addresses both issues. It doesn't matter which one comes first for operation. But to maximize headroom, if your voltage input is closer to the supply, drop it with NPN, then boost with PNP; and vice versa.

If the PNP and NPN transistors are similar in specification (size, power, bandwidth), they will track well. If they are located near to each other and have similar thermal environment, they track better. Even better is to use a dual package (BC846B or similar) that contains one of each. Pictured here is the response of the 2N3904 & 2N3906) over a range of temperatures (-40 to +80C) that closely matches the simulation data pictured here.
I've gotten similar performance on the bench with this circuit.

Zoomed in, we see that the spread over four temperature steps is large for the 1st stage (green), but very small (aqua) with the 2nd stage buffer added, as compared to the original signal (Tan). Also the output voltage offset is reduced from 0.7V to ~0.1V

The discrete solution shown is a very rugged circuit and usable over a very wide range of input voltages, limited by the selected transistors. As shown, it easily works with a supply of 5 to 36V. For higher voltages, I like to use the similar parts: 2N5550 or 2N5551 NPN and 2N5401 PNP which can go up to 150V.

This is a favorite circuit for its simplicity and performance; it's a good illustration of real-world transistor application.