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 !!!!

 

So what are your initial thoughts about solving the project?

For instance what is the (maximum) efficiency of a class A amp?

Therefore how much power does your amp need to handle?

 

dude 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!!!!!

 

I would use something like the BD239 NPN Silicon Power Transistor. It’s more than adequate to achieve your 3 watts of output and it’s cheap. Since you are using it for power you will setup your circuit as a Common Collector (CC) arrangement with the input on the Base and the output on the Emitter. Connect your Collector to Vcc. Voltage divider Base biasing is the easiest way to setup your operating Q point.
You can find more help at www.MoreDat.com

 

dude thanks for ur suggestion.......but why cant i use common emitter.

 

You need to tell us what your attempt at solution is to obtain help here - we do not do your work for you.


So how about an answer to my first question - it was a hint designed to lead you further along the right path.

The configuration as common emitter or common collector will be determined by the required voltage inputs and outputs. I doubt a CC circuit will be suitable in this case since a transformer is required.

You are correct in listing (some) of the required circuit values to be determined for a CE amp.
However should the current gain be one of them?

It would also speed things up if you told us what other parameters you might need to know and perhaps were given.

 

thanx for reply dude.......actually some values i have to assume and some i have to get from datasheet and some i have to evaluate using formulas and graphs....

Now hfe(gain) i will get from datasheet.......

I have started brief working....i have divided problem into dc and ac analysis and i am separately doing these two......plz check the attached file and review whether i am starting off rite or wrong !!!

 

Because you are going in the right direction, but starting from the wrong place, I will try one more time.

Firstly you work backwards from the load.
Bias and emitter/collector resistors (if any are required) will come out in the wash


What is the load?

This will determine the properties of the required output signal.

What is the input voltage?

This will determine the required voltage gain.

Is it a requirement that a transistor be used or could you use an FET or a Darlington?
Is it a requirement that a transformer be used? : I see no transformer in your schematic.

I confirm I would expect a power rail in the region of 12 - 18 volts to be adequate, but this depends in part on the load.

FYI, 3 watts is about the limit you can usefully develop with a simple class A semiconductor stage, even at this level a transistor CC pair might be employed as I assume jtrent was suggesting.
The implications are, that allowing for inefficiencies, 5 watt transistors will be inadequate, 10 watt or above will be required.

Are you with me so far?

 

1)i have been given no info about the load by my teacher...she says output power is 3w....so it can be either across Rc or across any arbitrary load resistance Rl..ie Rl=100 Ohm

2)i think i have to calculate the input voltage by using Ac analysis.....
i dont know the input voltage...and i think Hfe or gain is given in the datasheet...so wats the role of input voltage in gain???

3)i have to use npn Si or Ge power transistors and that too single stage

4)the attachments i have provided are only for series fed...i dont know a hell about transformer coupled.....

5) i cant use multistage!!!!!

Look i have to assume some values like those of Vcc,R1 and R2 and values of capacitors........ but i dont know what to exaclty assume and what to actually calculate...and if to assume what must be the precise assumptions that would be valid for my design problem......

 

The load information is the most important part. What is the amplifier for? A 3W audio amp to a loudspeaker? A 3W radio power amp driver for an antenna? A 3W servomotor driver? or what

All are possible scenarios with their own characteristics. In particular the frequencies and impedences will be different.

It is a perfectly reasonable and legitimate question to confirm with your teacher if she has not yet answered this.

Perhaps she expects to to realise this as you work along and ask further questions? This would also be a normal and useful teaching technique.

Never be afraid to ask.

I expect that for a 3W class A, the transformer primary will form the collector load and the actual external load (say 8 ohms for a loudspeaker) will be connected across the secondary. An RF power amp might have to match an antenna of 50 to 300 ohms. A servo motor maybe less than the speaker.

This being the case an emitter resistor will be needed to limit the direct current through the transistor.

Do you follow this?

 

You might think this is a cheap trick but, if you assume a transformer load on the collector, the teacher can assume that the transformer secondary is wound correctly for the load.

I'd just go at this like an audio amplifier. Look at a Fender Champ 5F1. It does 5 watts with a vacuum tube. Put a transistor in, instead of the tube, add DC bias to get the idle current right through an emitter rersistor, and you're good to go.

 

the teacher can assume that the transformer secondary is wound correctly for the load.

Yes, of course.

But the important teaching item here is that any collector resistor is undesirable.

ishaan has correctly identified that there will be a different analysis for DC and AC and this is one of the very important differences.

A high collector impedance is required for AC, but a low one for DC. Hence the use of an inductive collector load.

ishaan can you see why?

 

Thanks for the reply!!!!

Actually to be honest i have done the experiment of class A amplifier in our lab in which i had to find the gain and efficiency of a class A amplifier usiing the Kit......

So in the kit they had load RL=1 K Ohm......So can we take Rl= 1 K ohm for our design problem or we have to stick to low values which are practically possible for loudspeakers,motors etc.

we were also given the task to design a basic class A amplifier on breadboard(no specific value of output power was given)....using CL100 and then we had to calculate efficiency and all dat!!!!

The circuit diagram of the breadboard design is given as attachment!!!

 

Umm...that drawing is not a Class A amplifier. There is no idle current.

 

i understood the concept of collector impedance and all dat....when we performed transformer coupled on the kit....the primary of transf was at collector side while transformers secondry was in direct contact of load Rl...
where Rl=1k.........

The question now arises if i have to obtain output of 3W......in case of series fed as well as transformed coupled ....is this 3W the power across load Rl......or across the collector resistance??????

And the transistor i am taking for my design problem must be included in multisim's database so that i can simulate it!!!!

Hell lot of work and no experience.....dat's my present situation!!!!!!

 

Umm...that drawing is not a Class A amplifier. There is no idle current.

This is the basic circuit........The one we actually made had a voltage divider bias......with emitter resitor and emiiter bypass capacitor!!!!!

 

dude thanks for ur suggestion.......but why cant i use common emitter.

Your power calculations are wrong. What you really want, if I'm understanding you correctly, is to deliver 3 watts of power to your output. That means you need to be calculating the power for your output load, not for your collector resistor.

You can use a commen emitter circuit if you place your load in the collector of the circuit, however common collector is more commonly used to achieve power gain.

 

Let us take the collector resistor at 1000 ohms for the moment.

What are the current and voltage implications of this decision at 3 watts delivered power?

 

so now to get power gain i have to design amp in Common Collector !!!! No probs.....it can be done.......but can u tell me what input voltage must i apply to get output of 3W........

 

RC=1000......

ICmax= Vcc/1000=18/1000=18 mA

ICq= 1/2 Icmax= 9 mA

assume B= gain=25

IBq= Icq/B = 9 mA/25 = 760 uA

rest are resistor values of which some can be assumed and others calculated by trial and error.....while capacitors wont effect DC analysis part!!!!!