Other Parts Discussed in Thread: TL072, LM833, , OPA1656, OPA1655, OPA2156
I've just started working with the OPA1642A, and I want to use it in a basic audio buffer circuit. I've done this with older parts for years (LM833, TL072, etc.). I'm seeing something unexpected with regard to the bias voltage. With no signal input, Vmid measures really close to V+/2. However, as I give it an input signal (1kHz sine), the bias voltage drops, presumably due to current flowing into the op-amp. With a 9V supply and a 6V p-p input signal, I measure a 1.4V voltage drop across R4, which reduces the bias voltage at the positive input.
This is unexpected, I don't get this with other op-amps. There's some voltage drop, but an order of magnitude less, and it doesn't vary so wildly with input signal level. Can someone help me understand what's going on here?
A few more notes:
- I'm trying to maximize the headroom I can get out of a 9V supply, that's why I'm using a rail to rail opamp. Because the bias voltage drops, the bottom of the waveform clips early compared to what I would expect.
- I need the 2M resistors in there, because I need a 1Meg input impedance.
Thanks in advance!