Introduction: Transistor Integrator

About: We are a group of hobbyists that are designing analogue electronic circuits.

This Instructable shows you how to design and make a transistor analogue integrator.

The integrator allows the cumulative amplification of small input signals.

This circuit is obsolete and can be made with operational amplifiers.

However, you can still assemble it if you have spare general-purpose transistors.

Rf resistor needs to be adjusted because each transistor has a different current gain.


Parts: matrix board, wires, general-purpose NPN transistors - 10, general-purpose PNP transistor - 3, 1 mm wire, 470 nF pillow capacitors - 5, other components that are shown in the circuit.

Toos: pliers, wire stripper.

Optional parts: solder.

Optional tools: soldering iron.

Step 1: Design the Circuit

The first stage is the AC (Alternating Current) amplifier stage.

The second stage is the current mirror source integrator. I used a current mirror instead of a single transistor because I want to have a predictable charging current. Transistor current gain can change with temperature and collector current.

The voltage across the capacitor C2 proportional to the integral of the current. In a transistor current mirror source the supply current stays the same regardless of load/capacitor voltage unless the capacitor is fully charged or transistor is fully saturated. Therefore:

Vc2 = (1/C2)*(Ic2*t/2)

C2 = C2a + C2b

Where: t = time (seconds), Ic2 = C2 capacitor current (Amps)

The C2 capacitors will not fully discharge if the input signal to the circuit is zero because the Q3 transistor will turn OFF when the Vbe3 voltage falls below about 0.7 V. However, the C2 capacitors will discharge enough to produce a zero Q3 transistor output.

Because I am using a current mirror source and the two transistors are OFF in the second half of cycle, if Vc1 is a sinusoid than average Ic2 = rms((Vc1peak - 0.7 V) / (Rc2a + 1/(j*2*pi*Cb2*f)))

Where: f = frequency (Hz), Vc1peak = Vc1 AC Amplitude.

RMS stands for root mean squared.

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The last and third stage is another AC amplifier.

The circuit works at a minimum of 3 V. However, you might be able to reduce the supply voltage to just 1.5 V if you reduce all resistor values. However, the problem is low voltages is that the input signal has to compete with the noise.

Step 2: Make the Circuit

I have modified the circuit and also this article. I replaced the old electrolytic capacitors with pillow capacitors. I also added a few transistors in parallel.

You can see that I did not use a soldering iron. However, you might need it.

Step 3: Testing

First graph: Sine wave

Second graph: Square wave

Third graph: Triangle wave

The circuit output voltage increases slowly when the input frequency is raised to about 50 Hz. Then I lower the frequency and the input voltage falls as you see in my testing results. This is due to the high pass filtering properties of the Q1 transistor AC amplifier.

However, it is not evident in my testing results that by increasing the frequency the output voltage will fall due to low pass filtering characteristics of the C2 capacitors (C2a and C2b). I simply decided not to bother with recording those graphs. This is because the capacitors do not have the time to charge.