MOSFET Preamp

In this article, I will talk about some of the needs I have for this amplifier, some of the decisions I made in choosing devices, and how it fits in this application.

I want to amplify the voltage of a signal, which is commonly called a preamplifier. The signal is from a PWM output from an MCU. This means only two values are important, logic high and logic low. This also means that amplifier linearity is not important. I also wanted the input impedance and voltage gain to be relatively high. Most importantly, I wanted the system to be very simple.

I knew an N-Channel Metal Oxide MOSFET would be most ideal for this application. They can have a very fast switching time and have a high input impedance. The question was, how do I choose which device to implement. Digikey made it fairly straightforward. They had a column for the Threshold Voltage. I knew the target voltage I wanted was 3.3 V. I needed Vth to be well below that. I found one with a threshold voltage of 0.4 Volts. BSH103,215

I looked at their datasheet, and saw a range of currents that was well beyond the scope of what I was trying to do. For a preamp, I don't necessarily need the current to be very large, in fact it is better if it isn't. I can limit it to a small value by including a large resistance.

This is what I wanted. When the input is 0, I want the output to have 16 Volts. When the input is 3.3, I want the output to be very close to zero. This is impossible with nearly all amplifiers, but with device comes very close to that possibility.

Notice that the Current is in Amps. Also notice, the further up you go in current, the further away the Voltage strays from zero. So, if I want the output Voltage to be approximately zero, I need to keep the current to a minimum, close to 1 mA.

Therefore, I choose this device, with a 16 kilo Ohm resistor connected to the drain, and 16 Volts connected to that as my amplifier.

One thing of concern is the maximum continuous current, the switching time, and the maximum power allowed. The current is well below that, so that isn't an issue, but it does limit other potential applications. The difference between when the input changes and the output changes is a major concern for PWM applications. The switching time is 20 to 27 ns, which means the fastest the PWM could switch is 50 to 37 MHz. The maximum power rating is 540 mW. The drain leakage current is 100 nA, the drain voltage at that time would be 16 V. This results in 16 micro Watts of power. At the other extreme, the current is 1 mA, the drain voltage is approximately 0.1 V. This results in 0.1 mW of power. These are both well within the max rated power rating, but more questions remain. What happens when the device switching? This can only be answered accurately by wiring up the device and testing it using an oscilloscope for current and voltage while in application.

This completes my design of a MOSFET preamplifier.

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Electrical Design Engineer, Minneapolis, Minnesota