MOSFET  POWER  FOLLOWER

Last updated:  August 2, 2009
 

INTRO

This project is assumed as a "starting project" for DIY fans. It does not have ambitions to be a "top high-end" project.

MOSFET follower has benefits of single-ended class A amplifier with common Source plus something more - less distortion, lower output impedance, higher input resistance and .. no (global) feedback. With one drawback - voltage gain is only +1, so it needs preamplifier able to deliver say 10Vrms. But sound impression is perfect, surpassing the common Source MOSFET circuit. You can see schematics that I have tested in Fig.1.

PROS:

• simplicity, easy as a DIY project (Do It Yourself)
• low distortion
• no (global) feedback
• lower output impedance compared to common Source circuit
• higher input resistance compared to common Source circuit
• crossover distortion eliminated
• wide bandwidth
• fast response
• no overshoot, no ringing

CONS:

• voltage gain is only +1 (in fact +0.98)
• runs pretty hot, needs very good heat sink (better than 0.25°C/W)
• high input capacitance of power MOSFET (about 1500pF)
• because of high input capacitance the preamplifier must have output impedance no higher than 1kOhm

DESCRIPTION

The circuit consists of N-MOSFET voltage follower T1 (common Drain) and current source T2 (NPN Darlington). Current source is set to 2.2 Amps. With 40V of supply voltage the circuit is able to deliver about 17W into 8 Ohm loudspeaker. But takes 88W from power supply anytime. Bandwidth (-3dB) is from 4Hz to 250kHz. Rise time is 1.5 us. Output resistance 0.16 Ohm. The circuit is very tolerant to different kinds of load.

Input resistance is 10 kOhm (R0), but can be increased up to 100 kOhm (R4) when omitting R0. Input capacitance remains high, about 1500 pF. For this reason, the preamp should not have higher output impedance than 1 kOhm to maintain high frequency limit about 100 kHz.

An input potentiometer can be used instead of R0. If the value of the potentiometer is 5 kOhm then the high frequency limit will be about 70 kHz. The power follower can be connected directly to the output of CD player and for reduction of volume the potentiometer 5 kOhm can be used.

PARAMETERS

• output power                   17W / 8 Ohm (supply voltage 40V, quiescent current 2.2A)
• total power dissipation     88W per channel
• frequency range (-3dB)   4Hz - 250kHz
• rise time (10% - 90%)      1.5 us
• output impedance            0.3 Ohm
• damping factor                 27  (for RL = 8 Ohm)

POWER SUPPLY

You can see the schematic of the power supply that I use in Fig.5.

PREAMPLIFIERS

Rod Elliott's DoZ preamp is used (see www.sound.au.com or http://sound.westhost.com). An article describing the use of this MOSFET power follower with Rod's preamp can be seen at http://sound.westhost.com/project83.htm as "Project 83". The schematic of the DoZ preamp (©Rod Elliot, ESP, see his pages, and you can also order PCB's for the DoZ from him..) modified for this purpose can be seen in Fig.2.

To fulfill the specs of Q1-Q3 transistors, a supply voltage +Vs should be 30V. An experiment shows that they will survive supply voltage of 40V, so it is up to you to make a decision.... It might be better to use BC547/BC557 as they are rated at 45V.

When interconnecting the DoZ and the Follower, it is necessary to omit the R0 resistor of the follower. This slightly modified circuit can be seen in Fig.3. The complete amplifier is shown in Fig.4. As shown, the gain is 3.2, so it will require nearly 4V RMS input for full power. To increase the gain, you can change the R4 of the DoZ down to 1k to obtain a gain up to 23 (27 dB). Then increase the value of C3 to 47uF. The gain can be calculated as G = 1 + (R5/R4).

DISTORTION

Distortion figure shows the harmonic distortion curve from 15 milliwats to 6 Watts at 1.8kHz and 8 Ohms for 7W version of the Follower with DoZ preamp. The distortion is mostly second harmonic.

PRINTED CIRCUIT BOARDS

PCB's for this project are not for sale now.

Contact: macura@centrum.cz

P.S. You may reduce the supply voltage or quiescent current if your heatsink is not good enough. 30V of supply voltage will give you about 10W/8 Ohm. R7 will be then used to trim a current through zener diode 3V to maintain appropriate quiescent current through R6. This current (the current of constant current source T2) can be easily measured - a voltage drop in Volts accross R6 resistor has the same value as the current through T2 in Amps, because of the R6 value (1 Ohm). In case you reduce quiescent current to 1.4 A (at 40V of supply voltage) you will get about 8W/8 Ohm.
 

IMPORTANT NOTICE

Take care about zener voltage of the ZD3V. I use the BZX83 type, 0.5W, 3V zener diode. This diode gives proper zener voltage between 3.0V to 3.3V. In case you use bigger diode (1.3W, 2W etc.), most likely you will get different zener voltage, resulting in wrong quiescent current of the follower. Always check the zener voltage in this real circuit!! The quiescent current equals (zener voltage - Vbe of T2)/R6. And check voltage across R6, as quiescent current is

Iq = V(R6) / R6   (in schematics of the Power Follower)
 

Pavel Macura    November 9, 2001. Last updated: August 2, 2009.
 

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