OPAMP questions

[Kevin Weddle]

I have it. The input capacitor causes the gain to increase by 20db per decade. This is because the capacitor produces a 20db per decade roll-off and it is placed as an emitter element. As the frequency is increased, the gain is higher because it is an emitter element. A higher gain means it is able to keep the 20 db.

 

[audioguru2]

Kevin,
The input capacitor does NOT cause the gain to increase. It simply rolls-off the low frequencies, below its "corner" frequency at 20dB/decade. The corner frequency is determined by the capacitive reactance and the source and bias resistors.
Below the corner frequency, the capacitor passes less signal because its reactance is high (making an attenuator).
Above the corner frequency, the capacitor passes the full signal because its reactance is low (like a piece of wire).

 

[Kevin Weddle]

The input capacitor is to ground. It is not in series like a differentiator. Do you understand the concept of the emitter element? This is sometimes the resistor R1. R2 is the feedback resistor. The fact that it is an emitter element is why it sets the gain.

 

[audioguru2]

Kevin,
What opamp circuit has its input capacitor connected to ground?
Input capacitors usually couple in series an AC signal source to the opamp input and bias resistor for noninverting, or opamp input resistor for inverting circuits. The low-frequency rolloff of an input coupling capacitor is like a differentiator.
What is an emitter element?
If you are talking about an inverting opamp circuit, then its gain is simply R2 (feedback) divided by R1 (input) plus the source impedance.

 

[Kevin Weddle]

I have seen many configurations of the opamp and am still unconvinced of a few things. I believe it should be operated with differential inputs. I believe it should have the correct output bias current. Anything else is cheating the operation. It should also have the correct input bias current which is easy to calculate.