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Short version: Why is C3 necessary in this circuit?

Explanation:

The schematic for the input buffer/amplifier circuit for xoscope is made up of subsections that are mostly broken down in the descriptions on the site:

xoscope buffer amplifier

  • C1: AC coupling, to remove DC component from signals
    • In notes, the author suggests moving this to after the impedance stage
  • R1, C2: Impedance, to avoid impacting the operation of the circuit under test
  • R2, D1, D2: Clamp to +12.7V, -12V, to keep input within spec limits of amp
  • R3, D3: -11.3V reference for clamp to avoid dropping below common mode limit of amp
  • R5, R4, S1: Switchable 1x/10x feedback divider
  • R6: Output amplitude trim

I haven't found an explanation, however, for C3, the 100pF across R2. The circuit this derives from (The Art of Electronics, figure 4.74) does exactly the same thing, once again with no explanation.

Apparently the utility ought to be obvious, but I'm new to earnest analog electronics. I can only assume that it's stabilizing something, but I don't know how the operation would suffer without it.

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2 Answers 2

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My guess is that the designer is bypassing the series resistor in an attempt to mitigate the frequency roll-off caused by the diode capacitances of D1, D2 and D3. D1 and D2 are reversed biased, so there's the drift capacitance, and D8 is forward biased, so there's diffusion capacitance. Added together in parallel they might combine to give (just a guess) 20 pF of capacitance. That forms a low-pass filter in combination with R2, and without C3 would limit the bandwidth to about 170kHz. The TL084 has a gain-bandwidth product of 4 MHz, and with a gain of ten would have a bandwidth of about 400 kHz, so in this configuration it can do better.

I guess the designer is assuming(?) that at high frequency the source will be current limited and that the op amps own internal ESD protection will handle things; this circuit seems mainly designed to protect the input if, say, it's connected to an audio amp putting out 40 volts p2p.

C1 should definitely be moved; R1 and C2 are there because they are to be matched to the source impedance of the probe, and C1 will mess up the response!

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    \$\begingroup\$ The Horowitz and Hill circuit this is based on uses LF411 as the amp. I'm assuming that the TL082 was substituted due to its availability at Radio Shack. Incidentally, the sample rate on the target sound card would be less than 100kHz (44 to 48kHz most likely) so the value may not be extremely critical anyway. \$\endgroup\$
    – psmay
    Jan 16, 2012 at 18:50
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    \$\begingroup\$ C1 should at least be made larger, so that its reactance is small compared to 1Mohm at audio frequencies. (0.01uF is ~800kohm at 20 Hz. 0.1uF would be better.) \$\endgroup\$
    – markrages
    Jan 16, 2012 at 22:58
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It will be to compensate for the opamps input capacitance and other capacitance from the diodes.

At high frequencies the couple of pF capacitance will from a low pass RC filter with the 47k resistor and cut the signal.
Bypassing it with a 100pF compensates for this.

You can see the results of a rough simulation:

Schematic without 100pF (R2 and C2 represent the opamp input impedance with estimated parallel capacitance)

No comp

Simulation of above

No comp sim

With 100pF added

Comp

Simulation - notice there is far less attenuation at higher frequencies.

Comp sim

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  • \$\begingroup\$ Very educational! I really need to get better acquainted with my SPICE software. \$\endgroup\$
    – psmay
    Jan 16, 2012 at 18:51

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