The simplest opamp on discrete elements

The discrete-element opamps are manufactured for high-quality audio. They look like this - a board or “sandwich” of two boards and two combs for soldering instead of an integral eight-output double opamp with a standard socket. Whether the sound improves after replacement is unknown. But if the OU on discrete elements greatly simplified and turned into a detailed layout, the tutorial will turn out great.

In this circuit, all transistors of the structure NPN - 2N2222 or 2N3403, structure PNP - 2N2907 or 2N3906:
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The result of assembling the circuit on the breadboard is shown on the CDPV.

In contrast to the integral OU, here you can see all the transistors without a microscope and name their functions. Q1 and Q2 - current mirror, tending to evenly distribute the currents between the transistors of the differential pair Q3 and Q4. Well, Q5 and Q6 is another current mirror that seeks to bring the total current through both transistors of the differential pair to current through the resistor Rprg.

In the initial state, variable resistors are connected to the amplifier inputs - one to a non-inverting one, the second to an inverting one. By connecting the amplifier's output to the voltmeter's input, try adjusting the variable resistors at the amplifier's inputs, and you will find that a change in voltage at the first input actually causes a change in voltage at the output in the same direction, and a change in voltage at the second input - the opposite result. Set the same voltages on both inputs, then slightly change any of them, and notice how dramatically the output voltage changes.

Making sure that the circuit works, try to connect the device using some standard OU switching circuit. Let's start with the voltage follower, for this you need to connect the output to the inverting input, and to the non-inverting input to apply adjustable voltage:



On the layout it will look like this:



Comparing the voltage at the input and output circuits, you find that they differ from each other by no more than a few tens of millivolts. This circuit is good if gain is required not by voltage, but by power. To amplify by voltage, you need to set its coefficient by adding two resistors to the feedback circuit. The voltage gain factor depends on the ratio of their resistances, if they are equal, this factor is two:



On the layout:



Of course, a miracle will not happen, and you cannot force the output voltage to exceed the supply voltage. But even in the range in which this circuit actually increases the voltage twice, you will find an inaccuracy of several tens of millivolts. You can choose a reasonable compromise between accuracy and power consumption by selecting a “programming” resistor in the range from 10 kΩ to 1 MΩ. Less than 10 kΩ resistor can not be set, because the current mirror transistors can be damaged by overheating.

Some integral op amps have pins for this “programming”. But usually this is not done, and in this case, the resistance of the built-in "programming" resistor is what the developer considered optimal.

By eliminating the variable resistor, adding capacitors to the input and output, and setting the desired gain with additional resistors, you can get a headphone amplifier.