Hi All,
I have got a few questions about how to get the desired output signal from the transistor which this signal will be going to another stage which is an envelope detector (using a germanium diode)and some amplification to generate a square wave at the final output (AM receiver circuit).
The aim at this stage is to get an output signal about 2 v p-p AC with 2 v DC from the transistor.
From the datasheet for the transistor BF199
Bmin=40 & IC min =7mA
IC max is 25mA so I tried to get about 9 to 12mA so to be on the safe side (avoid Sat or Cutoff)
From spice model the BF199 Transistor has B =122
My approach to find the right parameters, Please correct me if I made mistakes,
Writing kVL
Vcc-(I*R)-(IB*R2)-VBE=0 where I= (B+1) IB=B*IB
IB=VCC-VBE/R2+B*R
I selected R as 150 ohm
R2 =29K ohm
R1=200 ohm
I used B (hFE) as 90.
I found the I =IE=9mA
Vout=1.8 volts which its close to get the signal to swing over it
Problems:
My problem is when I do insert coupling capacitors I do get a non linear signal like shown in the attachments and some distortion also there is positive cycle is wider than negative cycle. I selected the coupling capacitor as 0.01 and 0.1micro farad as my frequency is 12MHZ to get a reactance XC nearly as zero which it is acting as short circuit for the Ac and open circuit for the DC.
Im not sure how to start correct these problems (i tried to change the cap values slighty but it doesn't work). Any help will be much appreciate
Thanks
I have got a few questions about how to get the desired output signal from the transistor which this signal will be going to another stage which is an envelope detector (using a germanium diode)and some amplification to generate a square wave at the final output (AM receiver circuit).
The aim at this stage is to get an output signal about 2 v p-p AC with 2 v DC from the transistor.
From the datasheet for the transistor BF199
Bmin=40 & IC min =7mA
IC max is 25mA so I tried to get about 9 to 12mA so to be on the safe side (avoid Sat or Cutoff)
From spice model the BF199 Transistor has B =122
My approach to find the right parameters, Please correct me if I made mistakes,
Writing kVL
Vcc-(I*R)-(IB*R2)-VBE=0 where I= (B+1) IB=B*IB
IB=VCC-VBE/R2+B*R
I selected R as 150 ohm
R2 =29K ohm
R1=200 ohm
I used B (hFE) as 90.
I found the I =IE=9mA
Vout=1.8 volts which its close to get the signal to swing over it
Problems:
My problem is when I do insert coupling capacitors I do get a non linear signal like shown in the attachments and some distortion also there is positive cycle is wider than negative cycle. I selected the coupling capacitor as 0.01 and 0.1micro farad as my frequency is 12MHZ to get a reactance XC nearly as zero which it is acting as short circuit for the Ac and open circuit for the DC.
Im not sure how to start correct these problems (i tried to change the cap values slighty but it doesn't work). Any help will be much appreciate
Thanks
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