Mosfet

[Pim Driessen]

Dear forum members,

I have a diagram of an input preamplifier which I want to control with mosfets as in the diagram.

But the gate / source maximum voltage is 15 volts, how is this feasible, there are always zener used for gate / source of 9V1 volts, but the voltage at the input is also a 9V1-volt and that is not good.
Does anyone know a solution.

Greetings,

Pim

 

[kubeek]

I don´t see any means of controlling anything, just one input and one output. Can you elborate on what the problem is?

 

[Pim Driessen]

This is still not working preamp, the input stage operates.

I want as output stage using mosfets, but the DC voltage which gives the input stage, is 28 volts when I connect to the gate of the MOSFET, the voltage between Gate / Sourse is higher than the maximum 15 volts, which is the problem.

Greeting,

Pim

 

[kubeek]

is 28 volts when I connect to the gate of the MOSFET, the voltage between Gate / Sourse is higher than the maximum 15 volts,

Why would that be? The zeners should prevent that from happening.

 

[Pim Driessen]

Let me start again,

The diagram is an amplifier that works well, with transistors in the output stage, tested with stabilized voltage and working properly.

Now I want to use Mosfets instead of transistors, is that possible ?.

If a zener applicable than the input voltage drops also to 9Volt that does not seem right to me, the input stage is no longer the same, the differential amplifier is not equal.

Greeting,

Pim

 

[Pim Driessen]

this is what I actually what it should be, without the zener diodes.

But how can this be achieved without affecting the input voltage.

Because I think the Mosfets without the zener diodes break.

How can that be fixed, thought himself to a voltage divider ?.

 

[kubeek]

Why don´t you try connecting mosfets the same way as they were in the original? That means N-type in the bottom, source pointing down...

 

[Pim Driessen]

You mean so?.

I just do not understand what makes the difference?.

 

[kubeek]

You still have sources going towards the output, not the suplly rails as in the original. When you do that at least the topology will be similar. Then you will need to setup proper gate voltages to get some reasonable quiescent current.

 

[Pim Driessen]

You mean so.
The Source to the power supply voltage?.

 

[kubeek]

Yes now that is basically equivalent in topology to the original circuit. Wonder why you decided to swap the output pair for mosfets?

 

[Pim Driessen]

I like MosFets in audio circuits.
In a Mosfet is still the source the output, that you can turn ?.
In a simulation program it run, but with parts I do not know, have never tried.

where source is located on the supply voltage +.
Indeed there is no difference anymore between gate and source, which is almost equal. 32 Volt Supply / 28 Volt Gate.

but will they work?.

Greeting,

Pim

 

[Pim Driessen]

Hi Mr Kubeek,
Is this what you mean?.

Greeting,
Pim

 

[kubeek]

Hi, all I can say is it might work. I am sorry but I don´t have enough time now to do any proper analysis, not even to sketch it in a simulator. I guess it is up to you now to try what happens.

 

[Pim Driessen]

ok I try, and let you now.

 

[Pim Driessen]

To my surprise, it works absolutely great, the only thing that stands out is the square wave at 20 Khz is different than the transistor 2sa1078 / 2sc2528 version.

A tendency to curve no overshoot, but at 1 kHz, it is a corner without rounding.
What also amazes the tile coupling capacitor 20 Pf who just can out because I see no difference, while it showed a considerable overshoot in the transistor version.

So it remains briefly consider what the best result will give listening to music.

But what I do not understand is that the drain is the output and the source is at the supply voltage

Can you explain it ?.

Sincerely,

Pim

 

[audioguru]

The output transistors are inverters so the amplifier has positive feedback instead of negative feedback.
The Mosfets are followers that do not invert, then the feedback is negative. The output at the source of each Mosfet follows the gate with a few volts difference (source followers). But if the output is shorted then the gate-source voltage can be high but is prevented by the zener diodes.

EDIT: there are too many schematics in this thread, some with the Mosfets connected as inverters and some with them as followers. When the Mosfets are inverters then the supply current will be high. When the Mosfets are followers then they will probably produce crossover distortion.

 

[Pim Driessen]

Hello Mr. audioguru,

Are the zener diodes connected in the right way ?.

Greeting,

Pim

 

[audioguru]

Are the zener diodes connected in the right way ?

No, they are backwards and conduct all the time, limiting the gate-source voltage to only 0.7V so the Mosfets will never turn on.
If the zener diodes have the correct polarity then both Mosfets conduct at the same time as a class-A amplifier. Most amplifiers are class-AB for low idle current and low heating.

 

[Pim Driessen]

Now I see what you mean, I adjusted the schedule.

But should they be in because, the voltage between Gate / Sourse amounts to only a few volts.

Sincerely,

Pim