Circuit diagram of a simple discrete operational amplifier

This article introduces a simple operational amplifier built from five transistors and two resistors. It may be helpful for anyone interested in learning how a transistor-based op-amp works internally.

Of course, the performance of this op-amp is far from impressive — it suffers from large offset voltage, high bias current, low open-loop gain, narrow bandwidth, and a poorly performing output stage. Any off-the-shelf op-amp costing 10 cents will outperform it. However, as a discrete-component circuit, its structure is simple enough to serve as a great learning tool for those who want to understand what happens inside an op-amp rather than treating it as a “black box.”

Most general-purpose transistors can be used to build this circuit. We selected 2N3904 and 2N3906 transistors along with two 1 kΩ resistors. The core circuit operates from a ±5 V power supply. A pair of 10 kΩ resistors provides feedback, configuring the circuit as a non-inverting amplifier with a gain of 2 (6 dBV).

The article includes LTspice simulation results based on the default transistor models, as well as real measurement data from the physical circuit. The measurements confirm that the circuit achieves the expected gain of 2. For open-loop testing, an isolation transformer was used to obtain the required measurements.

Please note that in the LTspice schematic, VOL is the test voltage source for measuring open-loop gain, and in the actual measurement setup, the secondary winding of the isolation transformer is connected to the same point.

I hope you enjoy exploring this circuit!