BJT Frequency Response!

Thread Starter

zaman999

Joined Jan 29, 2011
26
Hi! In the freq response of a bjt,we see that gain is low for lower values of frequency,at mid band freq gain remains constant and at high frequencies gain again decreases.

My doubt is
1.how the coupling capacitors lead to decrease in gain?? :eek:
2.why the gain is constant at mid band freq?? :confused:




Thank You.
 

MrChips

Joined Oct 2, 2009
19,384
You need to show your circuit. Series coupling capacitors constitute a high pass filter, cutting out low frequencies. Increase the value of the capacitor by x10 or x100 and repeat the frequency response test.
 

Thread Starter

zaman999

Joined Jan 29, 2011
26
Yes,i've included the circuit and the frequency response.
Why is the gain low around 100Hz and beyond 20kHz??
also in midband region gain seems to be constant?
 

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Audioguru

Joined Dec 20, 2007
11,251
I disagree.
The transistor has its emitter resistor bypassed by CE. Therefore its input impedance is so low that the circuit will have -3dB response at about 2544Hz, not 100Hz because the value of the input coupling capacitor CB is much too low.

The high frequency response is poor because the SL100 transistor is very old and has high capacitances.

The value of CC also might be too low to pass low frequencies to a load.
 

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