Why is the purpose of capacitor CS?

Question

Solving the circuit below and as part of that I'm figuring out the purpose of these different capacitors.

Coupling capacitors at the input and output to separate bias matters from the AC signal
Emitter capacitor C_E to bypass emitter resistance R_E in order to boost AC gain

What is the purpose of capacitor C_S? I can't seem to figure it out.

Where did you get the circuit? Didn't it come with any explanation? — Elliot Alderson, Jun 28 '19 at 17:51

Tony Stewart Sunnyskyguy EE75 · Accepted Answer · 2019-06-28 18:00:51Z

4

Capacitor Cs attenuates supply ripple from modulating base bias current and then getting amplified by gm since Re is bypassed by Ce for high AC gain.

Technically this improves the PSRR.

answered Jun 28 '19 at 18:00

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\$\begingroup\$ So could it just be considered like a decoupling capacitor the base bias "rail"? \$\endgroup\$ – DKNguyen Jun 28 '19 at 18:39
\$\begingroup\$ Vcc + ripple - RC LPF = Vbias.. exactly \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 28 '19 at 18:40
\$\begingroup\$ Is R11 there for the Cs? I don't see why it would be there if Cs was not there since I see no reason why it should split the supply for RC and R12 otherwise. Actually, I'm not sure why you can't just have Cs without R11 either. \$\endgroup\$ – DKNguyen Jun 28 '19 at 18:51
\$\begingroup\$ To filter 100 Hz by -40dB say , X(Cs)/R11 = 1% thus to use smaller Cs you need bigger R11 at this low frequency cutoff of 1Hz. so if R11 =20k then Cs= ? For RC=0.16. If R=1 what is C? 0.16 Faraday \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 28 '19 at 18:55

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