MOSFET pair amplifiers that work on saturation

[ren_zokuken01]

Basically, I have the problem of amplifying a signal to go rail-to-rail on 24V, but I'm stuck with an opamp with power supply no larger than 5V (not exactly rail-to-rail itself, but max output of 5V should be a reasonable upper limit). So I thought of using a MOSFET pair amplifier to amplify the output of the opamp.

Since I'm amplifying 5V to 24V max with MOSFETs, the MOSFET pair needs to be in saturation region, V_in > V_th; (V_in - V_th) < V_ds, but I've always thought that Class A to Class C amplifiers always work in the linear region...

If, in fact, MOSFET pair amplifiers work in the linear region, how does it amplify small signals as linear region demands (V_in - V_th) >= V_ds (hence no need to voltage amplify the signal in the first place).

What am I not getting here?

 

[audioguru]

A bipolar transistor is saturated when it is turned on hard. But a Mosfet is "linear" when it is turned on hard and is saturated when it is an active amplifier. Do you want the Mosfets as a low distortion amplifier or to switch on and off?
When you say "Mosfet pair" do you mean Mosfet complementary pair that uses an N-channel Mosfet and a P-channel Mosfet?
I have no idea of how you can use them as a 24V rail-to-rail amplifier when the input is only 0V to +5V.

 

[ren_zokuken01]

I meant like this:

**broken link removed**

 

[kubeek]

That arrangement has two problems: one is that not a lot of mosfets can handle 24Vgs, usually the limit is +/-20V. Second problem is that with there will very likely be a time during the switching from one to the other, where both mosfets will be on and there will be huge current flowing through them, also known as shoot through.
Usually you use a separate driver for each mosfet, and some provision to provide what is known as dead time between one mosfet turning off and the other turning on.

 

[alec_t]

You will need to use level-shifting circuitry between the opamp and theFET gates. Google 'level-shifter'.
It is also advisable to allow a dead period when both FETs are off, to prevent shoot-through.

 

[audioguru]

Yes, I see that the P-channel Mosfet is always turned hard on like a piece of wire and the N-channel is turning on and off shorting the supply when it turns on if it is a "logic-level" type.

Wait a minute. You have the Mosfets on a dual-polarity supply but their inputs are 0V and +5V so that will certainly not work since both Mosfets will always be turned on hard.

 

[AnalogKid]

First, let's clear up the question. Are you asking about boosting an analog signal (sinewave or some other arbitrary waveform) or a digital signal (square wave, PWM, etc.)? The answers are radically different. Separate from that, your initial statement about MOSFETs is incorrect. They don't "need to be in saturation region". Maybe that's best for a particular application, but it isn't a general rule about MOSFETs.

If you really are dealing with a rectangular waveform as in post #3, and the input is a direction control signal, not a PWM or other repetitive waveform, then this is a fairly simple problem that can use either MOSFETs or bipolar power transistors.

1. What is the peak motor current?
2. What is the frequency of the input control signal? If it is a direction control, what is the shortest time it will be in either direction before changing to the other direction?
3. Is the driver in your schematic an opamp or a digital logic device?
4. What is the peak-to-peak voltage swing of the driver device?

ak

 

[Tony Stewart]

1st define voltage gain, then Impedance of dynamic load using only the motor coil resistance to see what is needed in between.

2nd define what is maximum power of motor and inertia of load.
3rd define what efficiency you want for motor control.

Keep in mind this schematic is just a simple concept and might only work as is for low voltage logic level drive and 5V motor as when both FETS are ON, this will short out the power supply without special bias or independent drivers are used to prevent "crossover" conduction or shoot-thru.