A POWER MOSFET INPUT AUDIO PREAMP

Terry Ritter

2005 September 24

Here is a tested design for simple, low-noise, 3-transistor audio preamps with a power MOSFET input. This design has various advantages:

It is RFI resistant.
Negative feedback provides for a fixed known gain and reduced distortion.
The input is biased at ground potential, which can eliminate the need for an input capacitor.
It uses only a single 9V supply at low current.
It operates as low as 4V.

The Power MOSFET Schematic

The input transistor is a power N-channel MOSFET. The gate is biased automatically by an op amp integrator "servo" that drives the output to some fixed level. This allows most N-channel power MOSFET devices to be used interchangeably, and directly compared in this service.

Power MOSFETs vary with respect to gain and noise level, so best operation requires some selection. However, other than exceptionally bad noise in particular devices, a wide range of devices function surprisingly similarly.

The input resistor (from the FET gate to opamp output) just sets the gate to bias potential and so biases the MOSFET. The value should be at least 10x the signal source impedence, but probably could be 100 megohms or higher. However, the bias resistor does have to charge the gate capacitance before operation, so an extremely high resistor could produce a start-up delay. Larger resistors do produce somewhat more Johnson noise, but that is normally "shunted" or "shorted out" by a low-impedence signal source.

The second transistor is a bipolar PNP. One might think to make it a low-noise low-current-linear device, but in practice that does not seem to matter much. Virtually any PNP device can be used with no circuit change and remarkably little difference.

The resistor across the base-emitter junction of the PNP sets the "standing" or "quiescent" current for the input transistor. The PNP operates on the "knee" of turn-on, at about 0.6V. The recommended value of 5K thus allows 0.6/5k = 120uA to flow before the PNP starts to turn on, which then increases the output voltage and starts to turn off the input transistor. This low current allows the MOSFET gate to operate at a bias voltage much lower than the documented "Cutoff Voltage," thus making a wide range of devices usable. The best current for lowest noise can easily be checked by varying the resistor value.

The output transistor is a bipolar NPN as an emitter-follower, and is also noncritical. The emitter-follower output does reduce the maximum positive output by 0.6V. So if we expect to work well with a 4V supply (a 9V battery at end-of-life), we ideally want the output bias to be around 1.7V, although we can vary this with minimal impact.

The output pull-down resistor provides the energy for negative-going parts of the wave. Since the emitter-follower can only source current, this resistor will define the maximum load which can be driven in a negative direction. Thus, it should be small, typically under 1/10 the input resistance of the next stage. However, this resistor also is one of the main contributors to power consumption, so it also should be large. If the output bias is set (by selecting the FET) at 1.7V, a value of 2K will sink about 0.85mA which the battery must then supply whenever the preamp is operating.

The op amp is an LM358, which functions well at low voltage, takes low current, and is very cheap and available. A dual device, we bias the unused section for a LOW output. The servo is a standard integrator with long time constant, which drives the bias toward a reference value. Originally the reference was 1.5V developed from three signal diodes. But that meant having both and input and an output capacitor in the signal path, which seemed sad. We might avoid an output capacitor by driving the output at ground potential, but then we need a negative supply so our signal can go negative.

We can develop a "negative" supply by calling the 1.5V reference "ground," and allowing the battery to float. This is a little tricky, since the copper "ground" is no longer the most negative potential. For one thing, the gate of the reverse-voltage-protect P-MOSFET can no longer connect to ground, but must be lifted to a pad to connect to the negative supply.

The Power MOSFET Board

Power comes in at the top. First there is a battery-protect resistor (47 ohms). If there is a board short, the resistor probably will die unless the battery is disconnected quickly.

Easy, cheap, and effective reverse-voltage protection is provided by a P-channel power MOSFET used backwards.

The input transistor connects at the left middle, the PNP is at the top left, and the emitter-follower NPN at the top right. Here each is in a little terminal strip so that different devices could be tried fairly easily. The op amp system is at the bottom. The blue component in the middle is a cubical 1uF film capacitor on the input.

Terry Ritter, his current address, and his top page.

Last updated: 2005-09-25