LOAD RESISTOR IN A MOSFET AMPLIFIER

[shaiqbashir]

Hi Guys!

well! i have designed a mosfet amplifier. This amplifier uses only MOSFET in its operation along with an input signal and capactors and resistors and VDD. I want to ask u one thing.

The amplifier which i have designed takes 0.1V of AC signal as an input and provides a gain of 3.3V/V. it is best for a load of 10K resistor. If i want to connect an 8-Ohm speaker as a load in place of 10K resistor, what modifications do i have to make. Plz help me.

Thanks a lot in advance

 

[audioguru2]

You need a power amplifier circuit or IC.

 

[Alun]

Could you please post the circuit?

It sound like you have a pre amplifier rather than a power amplifier to me.

 

[prateeksikka]

you can use a transformer with a turns ratio which is given by (N1/N2)`2=10k/8.
take square root  and turns ratio is approx 35:1.connect 8ohm speaker to the secondary and all would be fine! ;D
comments from others?

 

[audioguru2]

Hey Prateek,
Calculate how little the power will be:
1) 0.33V squared divided by the 10k load is only 10.8uW. If you held the 8 ohm speaker directly to your ear you might hear something faintly. The transformer matches the 8 ohm speaker to the 10k circuit, but the power isn't increased. ;D

 

[prateeksikka]

hi audioguru!
All same upto the stage i explained and an additional power amp at o/p across the load.would that serve?
u r correct!
power would be less but impedance matching is also important.
power amp in the end would it serve the job?do u personally approve it?
;D ;D ;D

 

[audioguru2]

Hi Prateek,
Using a power amp with an input impedance of about 100k ohms, a transformer isn't necessary.
Impedance matching is never used in audio circuits, usually the load is a much higher impedance than the source to obtain the most voltage transfer. The output impedance of most power amps is less than 0.04 ohms for good voltage transfer and superb damping. ;D

 

[shaiqbashir]

Hi guys!

the circuit i have made, i have attached it. Now what i desire actually is to use an 8-ohm speaker. tell me is this the amplifier which can be used to drive an 8ohm speaker. if not than what should i do.

remember that the 8-ohm resistor is actually my load.
Plz help me and remember use only FETs in ur suggestions.

Thanks in advance!

View attachment 37197

 

[audioguru2]

Hi Shaiq,
Your amplifier doesn't apply AC across the speaker that it is designed for. It applies a fluctuating DC voltage and current that causes its coil to be offset in the magnetic gap causing distortion and possibly overheating it.
Your ouput Mosfet doesn't have negative feedback to keep its operating point in a linear region. A Mosfet with a high gate threshold voltage will be cutoff most of the time, while a Mosfet with a low threshold voltage will be saturated.

Audio power amplifiers use push-pull transistors with positive and negative supplies, or with a large coupling capacitor to keep DC out of the speaker. They use negative feedback to keep the operating point linear, to reduce distortion and to make their gain predictable.

This project in our projects section is about the simplest Mosfet audio power amplifier that I have seen. It uses complimentary N-channel and P-channel Mosfets in a source-follower push-pull configuration, positive and negative supplies and uses negative feedback.
http://www.electronics-lab.com/projects/audio/001/index.html

View attachment 37198

 

[prateeksikka]

hi audioguru!
that means we are interested in maximum voltage transfer from source to load than maximum power transfer in audio circuits.But why is it so,as far as i ve studied we always prefer max power transfer from source to load?
???plz do reply! i am surprised

 

[audioguru2]

Hi Prateek,
RF power amplifiers use impedance matching to reduce standing waves in the cable connecting the transmitter to its antenna, and to reduce resulting over-voltage or over-current in the output devices. Audio power amplifiers don't have standing waves in the connecting cable so they use maximum voltage transfer to avoid power lost if the impedance is matched, and to provide damping of speakers. ;D

 

[shaiqbashir]

Thanks a lot audioguru!

you have proved to be a  real helper for me.

But there is another humble request to  you.

That will u plz review the circuit which u have suggested. Only basics i want, Plz Plz.

I shall be very thankfull to u for this act of kindness.

Thanks in advance

 

[audioguru2]

Only basics i want, Plz Plz.

If you want a basic lousy amplifier with high distortion, you can replace the two current sources with simple resistors, the two differential transistors with a single one and remove the bias-current setting pot and its bypass capacitor and replace them with a piece of wire. Then it will be about as basic as it can get. I don't think that anybody would want to listen to it because it would sound horrible! :

 

[shaiqbashir]

Hi Guys!

thanks a  lot audioguru for ur help. I considered your suggestions of having a negaitve feedback in my amplifier and now i have designed one thing which i have attached here. But i want to ask a few things:

1) this amplifier gives best results with the voltages in mVs like 1mV 2mV etc. but it gives terrible results with the voltages like 0.1V-0.5V. which we donot want at all. we want it to work well with the inputs of 0.1V to 0.5V.

2)is it a true thing that the output voltage in this amplifier doesnot depend upon the load resistor. i mean that in my amplifier(below) i have connected a 10K resistor as a load one, but i have seen that when i change my load to some other resisitance like 1K or something else, the result also changes and clippings and distortions occur. Can u tell me any way to operate any resistance as a load in this amplifier.

Please help me as soon as possible.
Thanks in advance

View attachment 37201

 

[audioguru2]

Hi Shaiq,
Why are you using very high power IRF250 Mosfets in your very low power circuit?
They are rated at 200V and 30A. Your circuit uses a max of only 20V and 1.47mA. Their current rating is more than 20,000 times what is needed in your circuit!

The IRF250 datasheet shows its performance with a minimum of 1A, and its transconductance is rated at 19A, so it is impossible to calculate its performance in your low-current circuit. Its very high capacitance is modulated by its amplification and cuts frequencies above only about 1kHz in your circuit. It is totally inappropriate for your circuit!
2N7000 low power Mosfets would work better.
http://www.fairchildsemi.com/ds/2N/2N7000.pdf

Your 25uF C1 and C2 coupling capacitors are much too large for the 500k input resistance of the Mosfet stages. They will take more than one whole minute to fully charge!
1uF coupling caps would pass 50Hz very well and would take only about 2 seconds to fully charge.

Your circuit doesn't have AC negative feedback, so its amplification and distortion are very high. :'(

Of course the amplifier's load affects its performance. Using your common-source stages, the Moset is a very high impedance, and the output impedance is the same as the 3.6k drain resistors. You need common-drain source-followers and negative feedback for a very low output impedance that isn't affected by its load.

 

[Alun]

How about using jugfets for the current sources, It'd be a lot cheaper and easier to build.

Just one comment about source followers as oppsed to bipolar followers, the turn on threashold on a MOSFET is higher than the 0.7V on a bipolar transistor so the voltage swing will be lower for the same amplifier.

What are the advantages of MOSFET amplifiers apart from the lower drive current?

 

[audioguru2]

Just one comment about source followers as oppsed to bipolar followers, the turn on threashold on a MOSFET is higher than the 0.7V on a bipolar transistor so the voltage swing will be lower for the same amplifier.

Hi Alun,
Good point! ;D

What are the advantages of MOSFET amplifiers apart from the lower drive current?

1) Better linearity at high currents.
2) No thermal runaway.
3) Much better high frequency response for low phase shift and more negative feedback.

I have made and tested many good high-power audio amps. The best have distortion at full power and less of about 0.01% over all audio frequencies. Some use ordinary transistors and others use Mosfets, so the type of output device is only a small part of their good design.

A pretty good audio amp could be made simply with an OPA445 high-voltage wide-bandwidth opamp driving some paralleled compimentary Mosfets.
Years ago I saw an amp design with a very good but restricted to + and - 15V supplies opamp driving a complimentary darlington push-pull output stage that had a gain of about three. I wish I kept the details. ;D

 

[shaiqbashir]

Hi Guys!


thanks a lot for ur help audioguru. But still i want to ask u one thing.

I have read that the drain followers are used when u need a very little voltage gain and i need a high voltage gain that is why im using source followers, then why are u compelling me to use drain followers.

secondly, how is it possible to operate the same circuit with an input signal from 0.1V to 0.5 V.

how should i make my output impedance so small that the same voltage gain is produced either with the 10K load or 8ohm load.

Please provide me details if u like. im in extreme need of help,
Thanks a lot in advance.



And how can i make that i

 

[audioguru2]

I have read that the drain followers are used when u need a very little voltage gain and i need a high voltage gain that is why im using source followers, then why are u compelling me to use drain followers.

What is a drain follower? There are common-drain amplifier stages with a lot of gain that you are using but with a high output impedance, or source-followers (common-drain) with a gain (loss) of slightly less than one but with a low output impedance that you are not using.
Why not use both, and add AC negative feedback to make the gain predictable and the output impedance extremely low? All opamps and good audio power amps do that.

How is it possible to operate the same circuit with an input signal from 0.1V to 0.5 V?

I assume that your input is 0.1V RMS, which is 0.28Vp-p. With your 20V supply, the most output possible is 20Vp-p. Therefore the gain required is only about 71. A single common-source stage can give a gain of 25, and two would give 625. Then add a complimentary source-follower output stage for a low output impedance. Adding AC negative feedback would reduce the gain to 71.4, reduce distortion and provide an extremely low output impedance so it can drive a speaker.

How should i make my output impedance so small that the same voltage gain is produced either with the 10K load or 8ohm load?

Source-followers have a low output impedance. AC negative feedback makes the output impedance extremely low, so that when you load the amplifier then the output remains the same. ;D