Electronics > Beginners
AC Coupling and OP Amp Stages
floobydust:
I think there might be a misunderstanding about the op-amp's operation when it is level-shifted.
Usually textbooks have the (+) input grounded at 0V for the inverting configuration, with perfect world +/- rails.
The op-amp stage has a voltage gain of around -5 in comparing the difference between the (+) input which is level-shifted about the Vref center point at 4.5V, and the (-) input before the 1k resistor.
What would be expected as an output when the emitter-follower is providing say 4.5V? 4.5-4.5=0V difference and the op-amp outputs 4.5VDC which is fine.
But with 4V out of the emitter-follower, the op-amp's output is shifted up near the rail at 7V, as it tries to maintain 4.5V at the (-) input with a single 9V supply. So any DC offset away from the Vref point gets amplified which can cause saturation, so an input capacitor is always used to block DC.
I'm not the greatest at explaining this, some people like math, others get a feel for it. I don't know your perspective.
eev_carl:
Does a set of op amp stages that are level shifted need coupling caps between each stage then? We're blocking the DC that was added in the + terminal. Is this positive feedback?
Benta:
--- Quote from: eev_carl on November 20, 2018, 09:55:55 pm ---Does a set of op amp stages that are level shifted need coupling caps between each stage then?
--- End quote ---
You've now asked this question in different ways several times in this thread. And you've been given the answer every time.
What you need to do is to set the DC bias voltage in the first amp correctly. Then the following amps will also sit at this DC bias point without coupling capacitors.
Your input emitter follower brings nothing, remove it, and run the opamp as a non-inverting amplifier as previously described.
eev_carl:
Does this look correct then? I ditched the emitter follower. The role of the first op amp is to provide a buffered signal that will bypass the second (and third) op amps. A JFET switch will toggle between this clean tone and the processed tone which is clipped and filtered.
Benta:
Should work perfectly. You now have a circuit with a gain of appr. -5 and a DC bias at V+/2. You can add following stages without coupling caps.
One comment: I don't know the LT1001, but 500 kohm is pretty high for a bias resistor. It depends on the opamp input bias current.
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