Electronic – Transistors on opamp power rails

Good question!

There is no ground leg of an opamp: there's a positive and a negative supply rail, and the current through each of them is roughly as follows:

$$\begin{aligned} I_{s+} &= I_q + \max(I_{out}, 0) = \max(I_q, I_q+I_{out})\\ I_{s-} &= I_q + \max(-I_{out}, 0) = \max(I_q, I_q-I_{out}) \end{aligned}$$

where Is+ is the current into the + supply terminal, Is- is the current out of the - supply terminal, Iq is the quiescent current specified in the datasheet, and Iout is the current flowing out of the output.

So, for example, if Iq = 1mA and Iout = 3mA, then Is+ = 4mA and Is- = 1mA; if Iq = 1mA and Iout = -3mA then Is+ = 1mA and Is- = 4mA.

This is because opamps are linear and use pass transistors in their output stages: positive output current = sourced current has to come from the + supply terminal, and negative output current = sink current has to go into the - supply terminal.

I say "roughly" because there are two other factors:

1. Input currents are never exactly zero -- if they're picoamps or nanoamps you can probably ignore them, but if the opamp is a bipolar opamp and the output is saturated (inputs are unequal) then the input current could potentially be larger than the specified input bias current, and could be significant enough that you can't neglect it. You'd have to know which polarity the input currents are: with PNP input stages, input current would be negative (current coming out of the inputs) and therefore add to the current from the positive power supply terminal; with NPN input stages, input current would be positive (current going into the inputs) and therefore add to the current exiting the negative power supply terminal.

2. Quiescent current isn't exactly constant, and can vary somewhat with operating point (especially if the opamp is in saturation, or if the output load is large).

What happens if there are resistors in those legs?

Then your supply voltage will vary somewhat due to the IR drops, depending on load current. Don't do this without two things:

1. You must understand completely what the load current is, and make sure it doesn't cause enough IR drops to make a significant difference in the circuit capability (e.g. op-amp saturation range contracting, or worse yet, opamp failing to work at all because supply voltage range drops below that for which it's guaranteed to work).

2. For goodness sake, put bypass capacitors between the supply voltage pins to your signal ground, so that AC supply current variation won't cause AC supply voltage variation that couples into your circuit.

Yup, opamps will pick up some of the signal on their power and interpet that as input. This is exactly what the power supply rejection spec is there to quantify. A ideal opamp would have inifinite power supply reject, but actual opamps have rather less than that.

Even with the power supply rejection spec, you can't take it at face value. It is very rare that the freqency is specified, so assume it is at DC only. The active circuit in the opamp that allows it to be relatively immune to power signals only works over some frequency range. It may be less susceptible to power signals at high frequencies because the overall response of the opamp falls off with frequency, or it may be more susceptible because the active rejection circuit can't deal with the high frequencies as well.

Whenever you are amplifying small signals by a large gain, you have to filter the power supply. This is standard practise. Even a little power signal into your small signal can make a large mess downstream. For sensitive amplifiers, such as when amplifying a microphone signal, always filter the power supply.

In your case, put a ferrite chip inductor in series with the power lead to the opamp, followed by a substantial ceramic cap to ground. Something like a 1 µH or so chip inductor followed by 10 µF to ground is usually good enough. This will form a L-C filter that strongly attenuates the high frequencies. These chip inductors usually have a few 100 mΩ resistance, which forms a R-C filter with the capacitor that usually attenuates lower frequencies before the L-C kicks in. For really sensitive circuits, put two of these filters in series on each power feed.