single transistor amplifier - effects of an emitter bypass capacitor on bandwidth

Thread Starter

Coollestersmooth

Joined Dec 18, 2014
72
I just wanted to check i am right in my thinking regarding the effects of an emitter bypass capacitor on the bandwidth of a single transistor amplifier.
from what i understand the lower bandwidth limit is set by the impedance of the bypass capacitor in parallel with the emitter resistor, however i am uncertain when it comes to the upper bandwidth limit.
Am i correct in believing that it is due to the impedance of the bypass capacitor being connected in series with the impedance of the base emitter junction?
 

GopherT

Joined Nov 23, 2012
8,012
Voltage Gain of a transistor stage is essentially a ratio of collector impedance divided by emitter impedance. The capacitor allows a lower impedance (more gain) as frequency increases.
 

t_n_k

Joined Mar 6, 2009
5,447
The main determinant of upper bandwidth frequency would be the Miller Effect rather than the emitter bypass cap.
 

MrChips

Joined Oct 2, 2009
20,263
The emitter resistor provides negative feedback and hence reduces the AC gain.
Thus if you bypass the emitter resistor with a capacitor you are going to restore the gain at higher frequencies. Hence it lowers the gain at low frequencies.
It determines the low frequency limit, not the high frequency limit.
 
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