Noobie Questions

[JJWright]

Hi People,

I was just looking at an ESP guitar amp design with op amps (project 27) - I understand that resistors 1k and 68k set the gain and was wondering what the 33pf cap, the 1uf cap and the 4.7k are doing to the circuit.

Any help here would be welcomed!

Regards

Jamie

[mchambin]

Bandwidth limiters.
the 33pf cap is to cut off high frequencies.
the 1uf cap and the 4.7k are to cut off low frquencies.

[sregor]

Gain is actually set by the 4.7k and the 68K resistors. The 1 uF cap is probably there for DC level reasons, but it does limit the low frequencies. The 1 K doesn't do anything - function would depend on what is connected to the other end. The output will have the same DC level as the input. As said before, the 33 pF cap is to cut high frequencies.

[JJWright]

Thanks guys - caps in the example are mainly for frequency then - my error on gain due to me mixing up non-inverting and inverting opamp equations I think. From the above example then av= 1+(r1/r2) =

15.47

Thanks

[wakibaki]

The 1uF cap isolates the opamp DC output from ground.

Opamp behaviour is such that the opamp output will be trying to maintain its voltage at a voltage which will result in the inverting input being at the same voltage as the non-inverting input when there is negative feedback.

Even if both the inverting and non-inverting input voltages are nominally zero, there may be a small but finite 'input offset voltage' required to maintain the output at zero volts, due to the small and differing currents which flow in the inputs (input offset current).

The cap reduces the gain at DC to 1 and means that no current flows to ground through the feedback network even if the input offset current is uncompensated. Were this not the case the uncompensated input offset voltage would appear at the output multiplied by the voltage gain set by the feedback network.

In order that the (AC) gain be maintained at greater than 1 at low frequencies, the reactance of the cap must equal the resistance of the (4k7) resistor to ground. This will cause the -3dB rolloff to occur at the frequency which produces this reactance.

w