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Designing feedback amplifier

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circuit is given.....problem is designing the circuit to given :
Av1=2,
Av2=2

Given data is;
RL=10K (RL=RE+RF)
fL=500hz
hie=4.5K
hfe=240
VCEQ1=VCEQ2=5V
ICQ=2mA

For any other data, parameters of BC147A can be used.

Requirement
To calc. the resistor and capacitor values to get reqd. gain.
(Av=Av1*Av2)

VCC has to be taken apropriately.
(Frequency generator source resistance is 50ohms)

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Hi Vishu,
A transistor amplifier? Why not use an opamp?

If Av1 = 2 and Av2 = 2 and you want the total gain to be 4, you don't want and can't use any negative feedback! The total gain will be a little less than 4 without feedback because the 2nd transistor circuit loads-down and reduces the gain of the 1st transistor circuit. Feedback or a load will reduce the total gain much more.
Where is RL? (it isn't shown)

If you want the gain to stay the same when you load the amplifier with RL, make the value of the output's Rc a lower value than RL to get a low output impedance (like an opamp) and make the gain of each transistor much higher than only 2, maybe a gain of 320 or more for a total open-loop gain of 100,000 or more (like an opamp). Then adding negative feedback to reduce the total gain to 4 will keep the gain constant when it is loaded (like an opamp).
My improved transistor amplifier will probably oscillate at a high frequency without adding a compensation capacitor (like an opamp has, built-in).

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• 1 month later...

HELLO VISHO,
ABOUT UR AMPLIFIER CCT U DONT TAKE IN UR ACCOUNTS THE LOADING EFFECTS FROM THE FIRST STAGE TO THE SECOND STAGE.
U WOULD BETTER USE TRANSISORS OTHER THAN OPAMPS, BECAUSE HERE U NEED ONE POWER SUPPLY BUT IN OP-AMP CCT U NEED TWO POWER SUPPLIES ONE -VE AND THE OTHER +VE.
BUT THE ADAVANTAGE OF THE OP-AMP ON THE TRANSISTOR THAT THE OP-AMP HAS AN INFINITE INPUT RESISTANCE WHICH IS NOT PRESENT IN THE TRANSISOR SO THE LOADING EFFECTS ARE NEGLECTED.
IF I WERE U VISHO I WOULD SAY THANX YOUSEF
BYE

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U WOULD BETTER USE TRANSISORS OTHER THAN OPAMPS, BECAUSE HERE U NEED ONE POWER SUPPLY BUT IN OP-AMP CCT U NEED TWO POWER SUPPLIES ONE -VE AND THE OTHER +VE.

Hi Yousef,
No, no, no, no! Opamps don't need two power supplies.
Single supply opamp circuits work exactly the same as dual supply circuits:

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Feedback creates a special condition of the voltage. You dictate the signal size onto the PN junction.In this case ,you are reducing the signal size at the PN junction. That is all that feedback is about.

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Kevin,
Forget about PN junctions, new Cmos opamps don't even have any PN junctions. Besides, an opamp is an extremely high-gain complete amplifier, not just a transistor. Since its gain is almost infinite, negative feedback keeps its inputs voltages nearly the same.

Non-inverting:
The non-inverting input of the opamp swings up and down with the input signal. The voltage-divider action of its feedback resistors makes its inverting input also swing up and down nearly as much as the main input (to keep both inputs the same). The gain of the non-inverting opamp circuit is the ratio of the feedback resistors plus one.

Inverting:
The non-inverting input is "grounded" and doesn't have any signal.
The input signal is cancelled (to keep both inputs the same) at the inverting input (virtual ground) by the inverted signal from the output. The gain of the inverting opamp circuit is the ratio of the feedback resistors, but only when fed from an impedance that is much less than its input resistor.

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Audioguru, just draw out some imaginary voltages around the opamp and let yourself be the judge.

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The voltages around an opamp are not imaginary, they are tried and proven!

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HELLO,
ABOUT YOUR COMMENT, YOU MAY BE WRONG BECAUSE IF YOU USE ONE +VE POWER SUPPLY THEN THE OP-AMP WILL NOT WORK PROPERLY BUT IT WILL SATURATE IN THE -VE REGION AND WE CANT GET A -VE O/P FROM THIS OP-AMP. WE WILL GET THE SAME THING IF WE USE A -VE POWER SUPPLY.
THANX VERY MUCH

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Hi Yousef,
NOT TRUE! The single-supply opamp has its input biased at half the supply voltage so that its input and output can swing equally both positive and negative with respect to half of the supply voltage. The opamp's input and output are capacitor-coupled to allow this. The "half supply" biasing voltage is bypassed to ground so that it becomes the circuit's ground.

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HELLO,
I AM SO SORRY, BUT I DIDNT GET IT YET!!
CAN YOU EXPLAIN THE ATTACHED CICUITS TO ME?? PLEASE
"I AM A BIGINNER.. FORGIVE ME"
???

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Hi Yousef,
I have added DC operating voltages to my opamp sketch that shows that all opamps can swing their outputs about 10V peak-to-peak or more with an AC signal.

If you don't understand how single supply opamps that are AC coupled can operate exactly the same as dual supply opamps that are AC coupled, then please learn about how opamps work in our Articles section.

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