Common Emitter amp questions

[rompelstilchen]

hello

i have been digging about BJT amplification and i got some questions, cos i am more confused after reading infos than before

1 - some say to calc Rcollector = Vcolector/Ic
some others say it is Rcollector = (Vcc-Vcollector)/Ic

2 - I found a tutorial that says to calc Vb :

R2 = Vb/(10*Ib)
R1 = (Vcc-Vb)/(11*Ib)

but i found a pdf that says the opposite (with the same R1 at Vcc and R2 tied to the ground)

R2 = (Vcc-Vb)/(11*Ib)
R1 = Vb/(10*Ib)

3 - for the gain :

A=-Rc/Re but others say A=-Rc/(Re+re) where re = 0.025/Ie

4 for the capacitors

for C emmiter : 1/(fmin*(Re/10)*2*pi) at some other place i found 1/(fmin*Re*2*pi)

for Cin : 1/(2 x pi x f x Zin) where Zin = (hFE+1)*Z emitter
but how do i calc Zemmiter if i have Re and Ce in parallel at the emmiter?

for Cout : 1(fmin*2*pi*RLoad) but what if i dont have any RLoad ? some say to try values between 1µ and 10µ but i am not sure about that

if anyone could enlight me about these details

thanks

 

[barry]

Rcollector = (Vcc-Vcollector)/Ic

IT'S IMPOSSIBLE TO KNOW WHAT R1 AND R2 ARE WITHOUT A SCHEMATIC!!!!

re:
The first equation doesn't include the transistor model (re). But I've seen it as 25/Ie, not .025

 

[rompelstilchen]

where cin is c1
cout is c2
and ce is c3


i read 25 mV voltage drop so 0.025V

and rc is r to collector, re is r to emmiter

i cannot attach my excell sheet on this site but i made all calculations
i tested it on a bread board, i got a output signal like a zillion times smaller thn the input (and completely distorted)

 

[barry]

What values did you test on your breadboard? Did you try simulating your circuit?

 

[rompelstilchen]

yes but a working simulation does not mean it will work on the breadboard

Rc=250
Re=60
R2=1.7k
R3=4.5k

For the capacitors, i cant get reasonable values, the formulas i found are realy wack and cannot correspond to a real case
Ce = 0.026F wich is enormous [ if i use 1/(2 x pi x f x (Re/10)) ]
Ce = 0.0026F [ if i use 1/(2 x pi x f x Re) ]
so i put a 1µF capacitor

 

[BradtheRad]

A capacitor is put in the emitter leg when you wish to amplify the AC signal. It is frequently done for radio frequencies.

The resistor is needed in the emitter leg as well, because without it the transistor could not perform in its DC function.

 

[rompelstilchen]

i know,

but that don't sove my problem

 

[FvM]

Rc=250
Re=60
R2=1.7k
R3=4.5k

Would you mind to use the same component references as shown in your schematic? We still have to guess "who is who". You also didn't tell about the supply voltage which matters a lot.

yes but a working simulation does not mean it will work on the breadboard

If you actually did the simulation, you should take the chance to compare the DC voltages achieved in both cases, particularly Vc respectively Vce. If Vc is somewhere around half Vsupply, the amplifier should work without distortion.

Selection of coupling capacitors comes next.

 

[barry]

yes but a working simulation does not mean it will work on the breadboard

Perhaps. But a non-working simulation can pretty much guarantee that your breadboard won't work either.

 

[BradtheRad]

Although many schematics have the bias supplied by fixed resistors, it really needs to be adjusted with a potentiometer, when you are experimenting.

That is the only way to find a good operating point. Otherwise you are liable to get too low or too high a bias.



The output waveform is clipped.

To obtain an undistorted sinewave, adjust the pot so it is centered roughly at Vcc / 2.

 

[paulfjujo]

hello


don't forget another parameter wich has a big effect on the result:
internal impedance of the signal generator !
All reactance of capacitor must be as low as possible at the frequency test...
1µF at 1000Hz Xc module = 159 ohms...


Test with C2=470µF accross 60 ohms => gain is near 50 with Rg internal of generator=1 ohms
but Gain decrease to 16 if Rg=500 ohms

Test Without C2
True EC with RE mode !
Approximation gives G=RC/RE for low gain and big h21e parameter of transistor.
but RC is resistor in collector circuit , in parrallel with the following R load
ex: RC=250 ohms and load=1Kohms => result RCequiv=200 ohms
G= 200/60 => 3.3 theorique 3.05 simulator

R6 is ajusted to obtain near V+/2 on collector