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[SOLVED] Cascade BJT AMP two stages - HELP

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darrylcapulla

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Hello Engineers,
I just want to Design a Cascade BJT Amplifier Two Stages,
but my problem is, Im only given two values Vin = 50mVp @ 25Khz and Vo= 3Vp,
I dont really know where to start.. They say that i need to draw a transfer curve to find the Value of Ic.
Please Help me. :cry::cry::cry:
 

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hi,
To be given only those three parameters is not sufficient information to design a realistic amplifier.

You should also be told what the requirements are for Input and Output impedance's are and the actual required bandwidth.

Where did the question come from, is it Homework.?


E
 

Of course you can design a two-stage amplifier with again of 60.
Because it shall work at 25kHz you need no further bandwindth information.
1.) Do you know the relation between quiescent current Ic and transconductance gm?
2.) Do you know the gain formulas for common-emitter stages?
3.) No supply voltage requirements?

I recommend not to distribute the total gain equally between stages because it is better to give the 2nd stage a lower gain (perhaps 6 or 10) .
In this case, you can design a second stage with a rather low output impedance (1k or lower).
 
Of course it possible to design a two stage basic amplifier with a gain of 60 at 25KHz, I said a realistic amplifier.

The suggestion for a 'low output impedance' is required is based on an assumption that a low impedance is required.
We are also not told the form of the signal input, as described Vin , could be sinusoidal, square or some other form.?

So I will say again: To be given only those three parameters is not sufficient information to design a realistic amplifier.
 
Yes, Its a home work.

Here is the samples of plates(-see attachment), i got it from last years homework..
There are Values that i really don't know where it came, like in Image 1.jpg found on plate no. 2.
I think that the Ic value, maybe its from the DC transfer curve connecting the upper left side to lower right, thats what i know to find the Icq.(Image 3.JPG) And i don't know how to make a graph of Transfer curve like that in Proteus so i cant find the Icq. :(

1.) I dont have any idea whats transconductance is.
2.) In plate 1 and 2, thats the formulas where using
3.) I think we can use any values, some are using 15V, 20V etc.

- - - Updated - - -

The Inside the red circles are the Values that i dont know where they came from.
Im making a carbon copy of Plate no.2, but the given is totally different. :cry:
 

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so I'm not going to do your homework for you, as this is some basic stuff you're going to need to be good at before you take more advanced classes. here's a decent website with some steps for designing 2-stage BJT op amps. it's mostly full of equations, but it's a start.

https://ampdesigns.tripod.com/2_Stage_BJT_amplifier.html

if you want an actual explanation, I recommend "The Art of Electronics". it's a good book which explains how amps are designed without getting too lost in the math.
 

darrylcapulla said:
1.) I dont have any idea whats transconductance is.
Click to expand...

That`s bad - it`s one of the most important transistor parameter ; it determines gain properties.

darrylcapulla said:
2.) In plate 1 and 2, thats the formulas where using
Click to expand...

But I couldn`t find any gain formula. So - you want to design a gain stage without knowing the corresponding gain formula?
 

hi darryl,
The circuit you posted has Ce1 and Ce2, emitter resistor bypass capacitors.
The impedance of these two capacitors must be considered in your Gain equations for 25KHz.

E
 
darrylcapulla said:
Sample of Gain Formula from my book
Click to expand...
Unfortunately, you cannot use any of the formulas as given in the attachements.
Why?
Because the formulas are rough approximations - valid only for circuits with effective ac feedback caused by an emitter resistance Re.
And exactly this is NOT th case for your circuits.
 

darrylcapulla said:
Hello Engineers,
I just want to Design a Cascade BJT Amplifier Two Stages,
but my problem is, Im only given two values Vin = 50mVp @ 25Khz and Vo= 3Vp,
I dont really know where to start.. They say that i need to draw a transfer curve to find the Value of Ic.
Please Help me. :cry::cry::cry:
Click to expand...

Normally there are many possibilities to provide Voltage gain at the expense of impedance ratios of load to input. But your question did not specify and often Rs=0 for simple design is not realistic.

It is possible to get this gain in one stage even with inductor on collector and CB feedback resistor to control gain , bias current but also lowers input impedance. LC resonance impedance /Rload ratio can also provide gain up to 1000 using impedance ratio of Collector to Emitter (rBE) Even more using gain using resonant gain impedance of infinite // R Load/rBE

Sample Design < Falstad SIM
2456329700_1418863652.png


A more complex calculation perhaps than a simple Class A CE amp but based on Load R to Re ratio with some gain with Q.=Xc/Rc
 
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