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# Confusing with Amplifing with tarnsistor

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#### baby_1

i created a circuit that can amplify a wave with 20mV amplitude.
i use bc639 transistor with the ic=1ma
i calculate:
VCE=6.25
RC=5750
RE=570
RB to ground=20K
RB to VCC=102K
VCC=12
Min beta=40
max beta=160
so average is beta=100

i see my input has been amplify with Gain=10 but when i add a bypass capacitor on RE resistor my output has disturbation,Why? how can solve this problem?

what is the capacitor value ?
at what frequency?
how do you test it?

Did you check the amplifier's bias point? According to my hand calculation, Vce will be very small, not corresponding to the intended value. Correct the base resistor network if necessary.

The nominal voltage gain will be 10 without, but about 200 with a Re bypass capacitor. What do you want to achieve.

Could you write your calculation FVM?

vce=6.25
rc=(vcc-vce)/.001=5750
we assume that ic=ie and ie resistance should be 1/10th of collector resistor so RE=575
VE=575*.001=.57
VBE=.6
VB=.57+.6
IB=IC/100(beta)
IB=10 Micro
Base resistance to ground is 9th IB
RB1=1.17/.00009=13 Kohm
RB2 to VCC
RB2=(12-1.17)/.0001=108.3 Kohm

is it correct? i see the Gaing=10 but when i add a capacitor to RE (Bypass) my output wave has disturbance...(Frequency=10Khz , Capacitor=100Micro Farad)
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i want to know when Capacitor has effect on output ? why it has disturbance?

Yes, RB1 of 13k sounds much better. As said, you should however check the real circuit VCE. One detail isn't exact in your calculation, you forgot to consider the RE coltage drop in the calculation. VCE would be 6.25 - 0.57 V, which is O.K. as well.

As said, the effect of the capacitor is increasing the gain. It's not particularly causing a disturbance, but increasing the (AC) output voltage above the circuit's capabilities.

baby_1

### baby_1

Points: 2
can you help on this , i tested some other resistor with bc639 but max Gain=100,
1-why capacitor has a bad effect on some transistor (for example bc337)?and start to disturbance?
2-when i decrease input voltage the diturbance removed? why?(20mv accuretly so much?)
3-how can we calculate the max input?

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