class A amplifier design!!!!

[ishaan3731]

i am given assignment by my teacher to design class A power amplifier that gives 3W output....
the circuit must be implemented both by series fed and transformer coupled techniques!!!!!

i have to:
1)implement the design problem on papers.......then find the efficiency also for both cases

2)design the circuits on multisim

Actually i dont know much about which transistor will be the best suited for this problem and will give output close to 3W....please suggest a transistor and also help me on how to start solving this problem as i am weak in designing !!!!

Also maximum efficency of series fed is 25% and of transformer coupled is 50%......... from datasheet i could get the dc load line and ac load line so that i can get values of Icmax,Vce(sat) and Hfe and determine the values of resistors...i am using voltage divider biasing........ so i need values of R1,R2(voltage divider network),Re,Rc and main is the value of Vin........(Vin is input signal) . i assume Vcc to be 18 volts!!!!!

 

[chuckey]

Your 3W amplifier is it going to produce 3W into a 4 ohms?, 8 ohms, 15 ohms loudspeaker? (or some different impedance.) With the transformer coupled amplifier, you can just "adjust" the turns ration to match the optimum load for your amplifier with the series transistor amplifier its not so easy, can you use an audio choke?.
Frank

 

[goldsmith]

Dear ishaan
Hi
You should tell more explanation about your out put and input impedance , and the frequency of operation ( it's band width ) .
And about your input signal ( it's minimum and maximum value of amplitude ) .
However , i can predict the dissipation range ! i think at this power , the efficiency will be awful and you can cook the egg on your last transistor !!
Best Wishes
Goldsmith

 

[ishaan3731]

1) i dont know which transistor to use........ and also my circuit is single stage (most proboably)
2)i dont know about the frequency of operation.......
3)the output of 3 w must appear across load Rl=1k ohm
4)actually at first my teacher asked me to use 2n176 transistor....which i later discuuses wid seniors and found that this transistor was a old one used in 1950's......its datasheet sucks and its completely obsolete...so i asked mam to give use freedom regarding choice of transistor!!!!
5) Now i also have o simulate on multsim.....or orcad...so that transistor must be included in multisim's database!!!!

 

[enjunear]

1) i dont know which transistor to use........ and also my circuit is single stage (most proboably)
2)i dont know about the frequency of operation.......
3)the output of 3 w must appear across load Rl=1k ohm
4)actually at first my teacher asked me to use 2n176 transistor....which i later discuuses wid seniors and found that this transistor was a old one used in 1950's......its datasheet sucks and its completely obsolete...so i asked mam to give use freedom regarding choice of transistor!!!!
5) Now i also have o simulate on multsim.....or orcad...so that transistor must be included in multisim's database!!!!

I think you either need to get direction (from the teacher or you) to define SOME aspect of the design requirements (freq of operation, or the transistor choice). Once that's set, the rest will get built around that choice. If the teacher is leaving it open-ended, then you should pick something (either pick the transistor and then determine the freq, or pick a frequency then determine a transistor).

You (or the teacher) have a decision to make before you can really move forward... pick a variable and put a constraint on it, then proceed.

 

[dselec]

https://www.electronics-tutorials.ws/amplifier/amp_1.html

 

[ishaan3731]

ahhh if u say lets fix the transistor .........let me take 2n5154.......... i have its datasheet wid me......let me take frequency 70 Mhz, Vce=5 and Ic=2.5 amps

 

[chuckey]

"3)the output of 3 w must appear across load Rl=1k ohm", so V^2/1k = 3 therefore V = 3000 ^ .5 = ~ 55. So you need at least 55V RMS = 150 V p-P. So if you have a RC coupled amplifier, the Vcc must be above this figure.!!
Frank

 

[goldsmith]

Again Hi
As Frank said , your supply ,should be above that voltage . and i don't think that this is reasonable !! . so , as i said , you'll be able to cook your food , on this transistor !!( it's efficiency will be very low ) .
And about your BW , how do you want to design , without knowing the BW ??!!! it can be RF or AF . it is important to select transistor and some other components .
And if your frequency is a bit high , you can decrease your supply , and increase the current and use simple Push Pull , or some other configurations .
Good luck
Goldsmith

 

[ishaan3731]

dude its just on papers and multisim......plz help me....give me a direction and then i will lead to it....i dont expect spoon feeding but just give me a firection so that first i start my paperwork and then simulate on multsim.....its not too much a big issue or project......its an assignment sort off stuff....... just show teacher that output is 3W on papers and multisim and she will hapily give u marks!!!!!!!

So please erybody......give me a direction to start with...... i just need 3W output...nothin else!!!!!

 

[chuckey]

We are trying to help. The transformer amp is easy, as said because you can effectively alter the load as presented to your amplifier by altering the turns ratio. The RC coupled amp is more difficult. As said for 3W into 1K, you need a Vcc of >150V, so choose 155V. Now the current into your load will be .155A p-p. So the transistor must be biased to a current of say 80 mA and 80V which is 6.4W, so you need a medium power transistor with a heat sink, with a collector(drain) load of 1K and a coupling capacitor suitable for your band width.
1. Find a suitable transistor.
2. Bias it to give the above DC static conditions.
3. Put in a sine wave and see what happens.
LOL
Frank

 

[goldsmith]

Again Hi
it seems a bit fun , because , he/she don't know , about the BW of that amp , and about max and min input signal amplitude and power and ... etc !!

---------- Post added at 23:49 ---------- Previous post was at 23:48 ----------

And i wondering how it is possible to design without knowing those parameters !!

 

[ishaan3731]

Ok if i have to use Vcc as high as 155V , rather i would make my circuit in multistage.......and also what if i reduce my load from 1k ohm to just 100 ohms....would then my Vcc reduce!!!!!!
Let me assume Rc=300 ohm.......transistor efficiency=60%.....Vcc=18 v....... so know i just need to find the values of R1 and R2 of voltage divider circuit , Re(emitter resistor) and the input AC signal to be applied at the base.

 

[enjunear]

Ok if i have to use Vcc as high as 155V , rather i would make my circuit in multistage.......and also what if i reduce my load from 1k ohm to just 100 ohms....would then my Vcc reduce!!!!!!
Let me assume Rc=300 ohm.......transistor efficiency=60%.....Vcc=18 v....... so know i just need to find the values of R1 and R2 of voltage divider circuit , Re(emitter resistor) and the input AC signal to be applied at the base.

You can answer part of that question yourself; chuckey gave you the math in his previous post:

"3) the output of 3 w must appear across load Rl=1k ohm", so V^2/1k = 3 therefore V = 3000 ^ .5 = ~ 55. So you need at least 55V RMS = 150 V p-P. So if you have a RC coupled amplifier, the Vcc must be above this figure.!!

Using those same equations, and RL(load) = 100, I get Vcc at something around 50V. From chuckey's math abov, 55V is RMS voltage, so Vpeak = Vrms*sqrt(2). Then you take Vpeak*2 = Vpk-pk, which is the absolute minimum for your supply voltage. It's just algebra; now you have all of the equations to walk from Pout @ Rload, back to Vsupply for a class A configuration. If I use RL=300, then Vcc is almost double the voltage at 100 ohms.

As for determining Re, the voltage divider (DC bias voltage for base, I'm guessing, since we have no schematic to tell where R1 and R2 are)... you should be able to determine them from the equations given to you in class, or from the book. The input level will be determined by what gain your amplifier circuit has... the higher the gain, the lower the required input power. That will be something you can trade off by properly tuning your amplifier to operate in class A.

 

[varunkant2k]

Hi, Please read this link. It is very easy to design Class -A but depending upon your requirement.
If your input is having frequency of fin, you can choose your amplifier bandwidth upto 10xFin.

**broken link removed**

---------- Post added at 18:45 ---------- Previous post was at 18:40 ----------

If you use DC supply, it becomes even more straight and simple to design. Your resistance divider, (you use for biasing) should a) limit current to <1mA range ( its totally your choice), b) Use resistance of minimum power rating that it can support the power dissipation in it (example, if 1kohm carries 0.1A, its power rating should be at-least I^2xR = 10W.

 

[chuckey]

varunkant2k, I attended a lecture by Linsey Hood in about 1972. It is still with me!!. He demonstrated that a 18 KHZ sinewave sounded different to a 18 KHZ square wave, so the ear must be able to pick up third harmonic of 18 KHZ, 54KHZ !!!! Don't understand why to this day, but it sounded different.
Frank

 

[enjunear]

varunkant2k, I attended a lecture by Linsey Hood in about 1972. It is still with me!!. He demonstrated that a 18 KHZ sinewave sounded different to a 18 KHZ square wave, so the ear must be able to pick up third harmonic of 18 KHZ, 54KHZ !!!! Don't understand why to this day, but it sounded different.
Frank

I've heard a similar demonstration with similar frequencies. Several of my fellow students and I came up with a bunch of theories... the best of which was that playing the square wave signal through the speaker caused it to stop performing as a linear transducer, so the speaker was not able to accurate produce even the lowest frequency, accurately. I don't know that we ever heard the real explanation, but that was the most plausible one that we came up with.

 

[BradtheRad]

A square wave is sudden transitions. A square wave doesn't carry through the air as part fundamental, plus some of 3rd harmonic, plus some of 5th harmonic, etc.

However a sine wave easily carries through the air molecules, being a smooth waveform.

A square wave is bound to impact our ears in a different manner. This assumes there is a transducer which can move quickly enough to reproduce a square wave. (Perhaps a piezo transducer.)