# Design a Voltage Amplifier

#### ThyristorWolf

Joined Aug 30, 2020
10
Hi guys.. I have to design a voltage amplifier..
The first task is to develop a voltage amplifier with an open loop voltage gain (with no attenuation
due to source resistance and no loading effect) of about 10 to 15 V/mV where 1 V/mV = 1000 V/V
gain. The input resistance of your amplifier should be around 2.5 kΩ while the output resistance
should be between 25 to 50 Ω. You are free to choose any analog electronic component available
in the market to develop your amplifier. Your circuit must be designed to operate within the ±10 V
power supply.

• The second task is to use your developed amplifier in feedback to provide a closed loop voltage
gain (with no attenuation due to source resistance and no loading effect) of 10.

i don't know how to start and where to start..
Can anybody help me in designing such thing with given parameters?
I will be very thankful..

#### ericgibbs

Joined Jan 29, 2010
12,200
hi TW,
Welcome to AAC.
E

#### ThyristorWolf

Joined Aug 30, 2020
10
i tried to design it on simulator for getting some close results..but didn't work...then i tried to work on calculations from dc analysis to ac analysis..but the i didn't meet the required gain as well as resistances..Kindly just guide me how to do it, i will do it on my own. Thank you..

#### ericgibbs

Joined Jan 29, 2010
12,200
hi,
Please post your simulator circuit, lets go thru it and find the problems.
We cannot really guide you, until we know your level of electronics knowledge.

E

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#### ThyristorWolf

Joined Aug 30, 2020
10
i made this circuit, but my resistance are not meet my requirements...

#### Attachments

• 14 KB Views: 25

#### Papabravo

Joined Feb 24, 2006
15,453
You have chosen to implement an NPN common emitter amplifier. I would like to ask if you are familiar with the expression for the voltage gain of such an amplifier. Then I wonder why the emitter resistor is not bypassed.

#### ThyristorWolf

Joined Aug 30, 2020
10
Hi p.
Oh sorry for that.. actually i attached bypass capacitor before but then i wish to get the result without it so that i can observe the difference and effect of bypass and without bypass...

#### Papabravo

Joined Feb 24, 2006
15,453
Hi p.
Oh sorry for that.. actually i attached bypass capacitor before but then i wish to get the result without it so that i can observe the difference and effect of bypass and without bypass...
Makes all the difference in the world. Your gain will still be limited by the load resistor in the collector circuit. Your bias point is also a disadvantage being located on the load line where it is.

#### ThyristorWolf

Joined Aug 30, 2020
10
Doesn't look much like voltage gain -- looks more like attenuation.
yes.. i'm thinking this too..
can you help me in correcting my circuit?

#### Papabravo

Joined Feb 24, 2006
15,453
Sure. Just tell me how you would compute the voltage gain of the Common Emitter Amplifier. Hint: it is a quotient with the numerator equal to the collector load resistor (500 Ω). Your task is to tell me what the two resistances are in the denominator, and how you can get rid of one of them.

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#### ThyristorWolf

Joined Aug 30, 2020
10
according to my knowledge, i will compute;
Av=-Rc/(re+Re)....(without bypass)
Av=-Rc/re......(with bypass)

if there is bypss then Re will be neglected as bypass becomes short..
am i correct...? kindly tell me

#### ericgibbs

Joined Jan 29, 2010
12,200
hi TW,
Did you study that 2nd link I posted in post #6, it shows how to design a voltage amp.?
E

#### Papabravo

Joined Feb 24, 2006
15,453
according to my knowledge, i will compute;
Av=-Rc/(re+Re)....(without bypass)
Av=-Rc/re......(with bypass)

if there is bypss then Re will be neglected as bypass becomes short..
am i correct...? kindly tell me
Yes, that is correct. Bypassing the emitter resistor is the key to getting reasonable gain from a CE amplifier. The operating or Q-point should be chosen at Vcc/2 to allow for the maximum voltage swing without distortion. I would try setting the emitter current at 1mA which should avoid saturation. Here is my CE simulation example. Gain is about 100 or 40 dB.

#### ThyristorWolf

Joined Aug 30, 2020
10
Thank you so much buddy, but how can i adjust my input and output resistance with the required values?
whenever i vary one thing, the whole thing becomes maze for me.

#### ericgibbs

Joined Jan 29, 2010
12,200
hi TW,
E

#### ThyristorWolf

Joined Aug 30, 2020
10
I'm using the above attached circuit,
i have to required Rin= 2.5k ohm(round about) and Rout=25 to 50 ohm..
which have gain of 10 to 15 V/mV. so firstly i am designing it on multisim..
Calculation:
Vcc=10V
Rin=R1 || R2 ||beta re
Rout= Rc || RL
RL=1kohm
Vin=0.5mVp
fin=1kHz
and the gain i am getting just round about 100 to 150
the required gain for me is 10,000 to 15,000V/V

#### ericgibbs

Joined Jan 29, 2010
12,200
hi TW,
Looking thru the 'spec' in post #1, it will need more than one transistor stage in order to meet the gain spec, etc.

It also states.
Your circuit must be designed to operate within the ±10 V power supply.

The second task is to use your developed amplifier in feedback to provide a closed loop voltage
gain (with no attenuation due to source resistance and no loading effect) of 10.

while the output resistance should be between 25 to 50 Ω.

So try adding a 2nd or 3rd stage.

E

Look here:
http://ampdesigns.tripod.com/2_Stage_BJT_amplifier.html

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#### MrAl

Joined Jun 17, 2014
8,061
Hi guys.. I have to design a voltage amplifier..
The first task is to develop a voltage amplifier with an open loop voltage gain (with no attenuation
due to source resistance and no loading effect) of about 10 to 15 V/mV where 1 V/mV = 1000 V/V
gain. The input resistance of your amplifier should be around 2.5 kΩ while the output resistance
should be between 25 to 50 Ω. You are free to choose any analog electronic component available
in the market to develop your amplifier. Your circuit must be designed to operate within the ±10 V
power supply.

• The second task is to use your developed amplifier in feedback to provide a closed loop voltage
gain (with no attenuation due to source resistance and no loading effect) of 10.

i don't know how to start and where to start..
Can anybody help me in designing such thing with given parameters?
I will be very thankful..
Hi,

Wouldnt it be simpler to use an op amp?