[email protected] (
[email protected]) wrote in message
I tried putting a capacitor from ouput to -ve input and it made it
worse! How come? More negative feedback should stop it?
Confused Student
Huh? You placed a capacitor from the output of the op-amp to where? What
is a "-ve input"? Since it made the oscillation worse I assume you placed
the capacitor from the output of the op-amp to the negative supply rail
(sometimes labeled something like -Vee, or othertimes just V- in the
datasheet). The negative supply rail isn't normally considered an "input"
pin on the opamp. Rather it is a supply pin while the inverting and
non-inverting input pins are referred to as the "input" pins. This type of
arrangement is precisely the type of load most likely to cause op-amps to
self oscillate for no apparent reason. A capacitor from the output to some
low impedance connection (IE: ground, negative supply rail, positive supply
rail) can cause problems. Normally a few tens of picofarads or less isn't a
problem.
For increased stability place the capacitor from the op-amp output to the
op-amp's inverting input pin (IE: from pin 6 to pin 2 on the 741).
Alternatively to, or in addition to this, place a small resistor (such as 47
ohms) directly to the output pin to the load. Both of these circuit
modifications will change the circuit's performance characterisitics,
sometimes in undesireable ways.