Mosfet Amplifier

[MNA]

Hello everyone,
please, see the attached circuit....

I m giving 2Volts peak to peak sine wave through the function generator ..... the third stage is a voltage buffer that should provide low output resistance ..... but when i m varying load (from 8 ohms to mega ohms ) , the output voltage decreases.....

My question is what is wrong in my circuit ? since without connecting load the output voltage appears to be 10-11 volts .....

the volatage buffer should provide low output resistance....Isn't it?

Thanx for any help and suggestion.....

 

[hentai]

hmm there is a mismatch of impedance between first and second stage. First has its output impedance much higher than the second stage input impedance... Those current mirrors need only one ref current... Also i'm not sure what u intended with the capacitor and resistor in the source of the first stage... also im not sure but do common gate amplifiers work well with a constant current source in their source?

 

[audioguru]

The output doesn't have enough class-A current to drive a low resistance load. It is a high impedance and needs to have a high impedance load.

A class-A audio amplifier has an output like that but has a few Amps of current in the current mirror so it can drive a speaker. It gets extremely hot with such a high continuous current.

Class-A audio amplifiers also have a high amount of negative feedback to help reduce the output impedance. Your circuit doesn't have any negative feedback.

 

[MNA]

Thanx for replying....

the voltage buffer provides low output resistance,Isn't it?

few ampere currents ?how much? is bs170 ok for the output stage?

 

[audioguru]

Your voltage buffer operates at an extremely low current so it has a very high output impedance and it cannot drive a low impedance load.

The current in your output transistors is only about 0.5mA so the max peak voltage into an 8 ohm load is only 0.5mA x 8 ohms= 4mV.
If you increase the current 0.5A then the max peak voltage into an 8 ohm load will be 0.5A x 8 ohms= 4V. A peak voltage of 4V into 8 ohms is an RMS power of only 1W.

If the output transistors have 0.5A through them continuously and about 20V across each one then they will each dissipate 10W and they will melt since the max power rating for a little BS170 is only 830mW.

If you want to make a power amplifier then you must use power transistors and use enough current.

 

[MNA]

Hello everyone,
please see the attachment....

Audioguru, i have now used irf540 and 1A current(although i think 0.7A was also sufficient) .....Is it ok now? Will it work ?..... 0.7A * 20 V = 14 Watt (it is okay for irf 540) ......

The problem i m facing in this circuit is that before the output stage the gain is nearly 10 ..... when a 100 ohm load or (100+ ohm load) is connected then there will be no loss of gain but with less than 100 ohm load the gain starts to decrease and for 8 ohm load the gain is just 4.2 ...... (1A * 8ohm = 8V ....so the current is not causing any problems then....) ....why the gain gets reduce to 4.2 ???

waiting for your reply.....
Thanx for any help....

 

[audioguru]

Now the amplifier's output transistors will get very hot all the time and the max power into an 8 ohm load is about 14W. The IRF540 will heat with about 25W so it will need a huge heatsink.

Look at the very small value of your output capacitor. At 2kHz it reduces the power to an 8 ohm load to half. At 1kHz the power to an 8 ohm load is 1/4. With a 100 ohm load then most of the 1kHz output power gets through the little 10uF capacitor to the load.

 

[Nigel Goodwin]

Is it just me?, or does this 'circuit' bear no resemblance to a real amplifier?.

 

[audioguru]

No it is not a real amplifier.
It doesn't have enough voltage gain.
It doesn't have any negative feedback.
I operates in class-A so it gets very hot.

 

[MNA]

Is it just me?, or does this 'circuit' bear no resemblance to a real amplifier?.

i don't know.... but what i know is that it won't operate correctly ...so definitely it is not a real amplifier......

i have put this here to ask how to make a 'real' amplifier ...... the attachment tells u about my understanding of the amplifier ..... and if u think it is not an amplifier so u might have guessed where m i lacking to design an audio amplifier...........

i tried it myself (i think it is always good to try yourself first as compared to making a post having a demand like "Post an audio amplifier schematic" etc...)

but your post makes me feel that i should have better ask for an amplifier schematic without trying myself.......

It doesn't have enough voltage gain.

how much will be sufficient? and how can i achieve that?
with a 20 volt supply 10v/v isn't sufficient?

It doesn't have any negative feedback.

how to give a negative feedback and what changes i have to make in order to apply -ve feedback?

Thanx

 

[Nigel Goodwin]

i don't know.... but what i know is that it won't operate correctly ...so definitely it is not a real amplifier......

i have put this here to ask how to make a 'real' amplifier ...... the attachment tells u about my understanding of the amplifier ..... and if u think it is not an amplifier so u might have guessed where m i lacking to design an audio amplifier...........

i tried it myself (i think it is always good to try yourself first as compared to making a post having a demand like "Post an audio amplifier schematic" etc...)

but your post makes me feel that i should have better ask for an amplifier schematic without trying myself.......

A simple google will find hundreds of audio amplifiers on the net, designing your own is a praise worthy aim - but you should at least look at some others to see what an amplifier looks like. Would I be correct in assuming this is supposed to be a power amplifier, and feed a speaker? - it's so completely nothing like that, that I'm only guessing that's what you're trying to do.

No disrespect, but the circuit above looks like you've never even seen an audio amplifier, and have no idea what it's supposed to do, never mind how it's supposed to do it.

how much will be sufficient? and how can i achieve that?
with a 20 volt supply 10v/v isn't sufficient?


how to give a negative feedback and what changes i have to make in order to apply -ve feedback?

You need to throw it away, and design one that has a chance of working - your design has loads of non-required parts in it (all the current mirrors), and is missing most of the required parts (such as a push-pull output stage, or any output stage at all).

For an example (first hit in google) try **broken link removed**

 

[audioguru]

Most audio power amplifiers have an open-loop voltage gain of at least 10,000 at low frequencies. Then they reduce the gain, distortion and output impedance with negative feedback so that a fairly low line-level input of 100mV will produce full output.

Somebody on one of these forums recently posted the schematic of a Mosfet amplifier that had excellent spec's without any negative feedback.