Twin-T Audio Oscillator

R1 = R2
R3 = 0.1 x R1
C2 = C3
C1 = 2 x C2
R4 = 3K3 (approx)
C4 = 50nF (approx)
Q1 = any NPN audio amplifier.
e.g. BC108, etc
Vcc is not critical, 6 to 12Vshould work fine. Higher or lower voltages may also work OK.

I am not sure of the calculations required to determine the values of R1-3 and C1-3, but below are the values that I have successfully tried, and the frequencies that they give. For the circuit to work reliably the ratios shown above should be used.

R1

R2

R3

C1

C2

C3

Frequency

18K

18K

1K5

47n

22n

22n

700 Hz

22K

22K

2K2

22n

10n

10n

1200 Hz

The output of this circuit should be fed into a high-impedance load, say >100K, remembering that the impedance of C4 is around 3K.

A PNP transistor can also be used, with the necessary changes to the supply voltage. An op-amp can also be used, connected as follows:

Op-Amp Transistor
Output Collector
Inverting Input Base
Non-Inverting Input Emitter

 

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