Log amp - why can't I stabilise it?

snipped-for-privacy@hotmail.com wrote: (snip) > I've today also been able to reduce the oscillation for the first time > by the circuit I mentioned here for opamp 1: >

It seems to me (hunch follows) that the oscillation is caused by both the gain and phase shift added by the feedback transistor. Some of that phase shift comes from the base capacitance of T1 which puts a pole in the frequency response of T1. If you parallel R3 with the right size capacitor, you can have the capacitive voltage divider of these two capacitances parallel the resistive divider of R3,4 and cancel this pole. This method increases the frequency response, and reduces the size requirement of the output to inverting input capacitors, which roll off the high frequency response.

Dear All,

Mark: the oscillation reduction seen when "isolating" the scope probe with a

Win: The input is 22n in series with 6k8..

John: Thanks for your suggestion. You are however looking at Ban's suggested circuit. I am not using this. I am using the circuit at the start of the thread with op amp 1 enhanced having a classic capacitive load drive arrangment. I cannot use Ban's circuit because it is not suitable for an a.c. input,

regards, Colin

Ouch, that's a bandwidth killer at low currents. You'll need to increase the first amplifier's feedback capacitance.