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Project Notes

#094 BJT Long-tail Pair

Test a BJT “long-tail” differential amplifier circuit.

Here’s a quick demo of the circuit in action:



Differential amplifiers are excellently described in w2aew’s tutorial on YouTube.

This is a build and test of the basic long-tail circuit with no frills, which means the circuit is not ideal because:

  • it relies on matched transistors
  • output voltage varies with input voltage levels
  • differential gain varies with input voltage levels
  • only has moderate differential gain

The circuit includes an LED pair and resistor load between the outputs. This provides red/green - left/right balance indicator.

What is immediately noted is that gain varies wildly given different input voltages, from approaching 0 at large voltage differences (clipped by the circuit I believe), to 10-100 for small variations.

There are two improvements suggested by w2aew that will bring this circuit closer to an OpAmp:

  • constant current circuit on the emitter, to eliminate the impact of input voltage on output voltage and gain.
  • current mirror on the collectors, to improve gain

I’ll try these improvements in another project file.

Monitoring with an Arduino

The Arduino program is used to monitor the input and output voltages, calculate the differentials and gain. Readings are made using 1MΩ voltage dividers, to limit impact on the circuit and to scale the voltages to the 5V range supported by the Arduino analog input pins.

Although not particularly accurate, it gives a rough indication of behaviour. For example:

                ao1: 2919 mV
                ao2: 2371 mV
                ab1: 1837 mV
                ab2: 1793 mV
 input_differential: -88 mV
output_differential: -1096 mV
               gain: 12.45



The Schematic

The Build

Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.