I am trying to analyse this amplifier circuit and I can not understand what the purpose of the 3 capacitors C1, C2 and C3 are if anyone could explain that would be much appreciated.

Thanks in advance.

A Multisim circuit of the LF351-based audio amplifier

  • \$\begingroup\$ Q2 is inverted and power is not grounded. C2 IMHO should not be there. \$\endgroup\$
    – fraxinus
    Dec 11 '20 at 22:33
  • \$\begingroup\$ C2 probably made sense before C3, Q1 and Q2 were added. Anyways, those at least are to "A/C couple" or "high-pass" the output. \$\endgroup\$ Dec 11 '20 at 23:00
  • \$\begingroup\$ Where did you get this circuit from? \$\endgroup\$ Dec 11 '20 at 23:13
  • \$\begingroup\$ similar design:- circuitdigest.com/electronic-circuits/… \$\endgroup\$ Dec 11 '20 at 23:37
  • \$\begingroup\$ A few websites in India have this circuit that is full of errors. If it had the missing -9V supply and ground between the supplies then it would work a little with some distortion and a massive shoot-through current in the darlingtons. Which darlington is upside down? \$\endgroup\$
    – Audioguru
    Dec 12 '20 at 2:01

The capacitors on the output are to remove any DC from the signal and provide some high-pass filtering.

C1, is a feedback capacitor and it can reduce the phase shift in the feedback that can be caused by the input capacitance and series resistance from the signal. It can add some stability to the amp.

  • \$\begingroup\$ thank you so much do you happen to also know what the purpose of R4 is because I understand the purpose of all the other resistors thanks. \$\endgroup\$ Dec 15 '20 at 22:41
  • \$\begingroup\$ @ChristoPolachan Unfortunately I don't know. Often you will see one like that but it will be AFTER the A/C coupling capacitor to set the bias point. It is not typical in this type of amplifier. It may be a mistake, or maybe somebody who knows more than I can chime in. \$\endgroup\$ Dec 15 '20 at 22:57

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