AC Coupled Common Emitter Amp Oscillation Problem

[Rakdren]

I have a square wave in a circuit where there should not be one; anyone know why? (see photo for info)

 

[BobK]

I am surprised that circuit would oscillate since it is not high gain and it is low input impedance.

It looks like it oscillates at about 700KHz. If you are not using frequencies that high, perhaps a low pass between the stages would help.

What is the upper limit of frequency you want to handle?

Bob

 

[duke37]

Looks like you have made a multivibrator, collector to base and collector to base. The coupling Q8 to Q7 will be via the supply which has no bypass capacitor.

 

[Rakdren]

Bob, This amp is used to amplify the signal of a 40KHz ultrasonic transducer (which was not attached for scope photo). Kicker is, of three circuits, two have the oscillation and one does not yet they are all built from the same schematic. I would appreciate your idea of a low pass filter (values and configuration). Thanks for responding.

 

[Rakdren]

Looks like you have made a multivibrator, collector to base and collector to base. The coupling Q8 to Q7 will be via the supply which has no bypass capacitor.

Do not understand the position of where the bypass capacitor would go in the circuit; could you elaborate?

 

[duke37]

The capacitor or capacitors should go between the V+ supply and the ground. Use 10μF and 0.1μF. Keep the connection leads as short as possible.