hi,
what is the advantage of using resistor capacitor series circuit across amplifier feed back resistor from output to bjt base. resistor value normally below 1000 ohms and cap value is around 15pf . lt spice simulation shows increased feed back above 100 khz.
Yep, that's there for a reason.
As frequency rises, so does the phase shift in the amp. If it has enough gain at the point where the output is 180 degrees out of phase it will oscillate. The cap helps prevent that by decreasing the gain at higher frequencies.
so increased high frequency feedback in negative feedback amplifier reduces gain of input stage for high frequency signal and force the amplifier gain below unity before 180 degree phase shift. am i correct?
so increased high frequency feedback in negative feedback amplifier reduces gain of input stage for high frequency signal and force the amplifier gain below unity before 180 degree phase shift. am i correct?
Close ... it reduces the closed loop gain before the phase shift in the feedback causes oscillation.
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so increased high frequency feedback in negative feedback amplifier reduces gain of input stage for high frequency signal and force the amplifier gain below unity before 180 degree phase shift. am i correct?
It cannot reduce the gain of the input stage. That input stage is not modified in any way so does not change in behaviour.
Jan
If we are talking about the cap often seen across the feedback resistor then this actually increases loop gain, but it also reduces phase shift. With the correct value this can improve loop stability; get it wrong and it makes things worse. It also reduces the overall amplifier gain at high frequencies.
100% correct but you have to really read it correct and understand the difference between loop gain and closed loop gain.
Jan
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