|Hawes Amplifier Archive||
|FET Preamplifier, Part 3|
Why an MPF102?
It's available. Other Web designs use the Siliconix J201 or Motorola 2N5457. These are fine choices, but you can't find them at your corner Radio Shack. The MPF102 is easy to find and should provide satisfying performance. You can buy one and get it working today. Better yet, suppose that your JFET blows while you're on the road. With some other JFET, you'd be out of luck. Not so with the MPF102. Instead, you can get a replacement anywhere. Just find a Radio Shack.
It's durable. Also, the MPF102 is just as sensitive as these other devices, but probably a little more rugged. A J201 handles a maximum of 1 mA of current. The maximum for the MPF102 is 20 mA. Since FETs are very fragile devices, maybe the MPF102 has an edge. Maybe it will stay with you for a few years. Who knows how long you'll have a J201?
Low-Z output. With an MPF102, the output impedance is lower than with a J201. A little more power on the output means that your signal's a little stronger. You can probably drive a longer cable with the MPF102 than with the J201. I haven't tested this idea.
How It Works
Power source. "Vdd" represents the power source for your JFET amplifier. The power source is a transistor radio battery. The battery provides nine volts to the drain of the JFET device, Q1. This voltage connects to the drain through drain resistor R2. The drain resistor is equivalent to the plate resistor in a vacuum tube circuit.
Common terminal. One terminal of this amplifier is common to both the input and output signals. This terminal is the JFET source terminal. For this reason, we sometimes call this amplifier circuit a "common source circuit." The circuit is equivalent to a grounded cathode tube circuit. Source resistor R3 connects the source to the battery's ground terminal. The source resistor acts like the cathode resistor in a vacuum tube circuit.
Class-A bias. The voltage drop across the source resistor also sets the JFET bias. Under no-signal conditions, bias voltage causes the JFET drain to draw a particular average current. This current sets the drain voltage at a point halfway between the Vdd voltage and ground. The name for this "halfway setting" is Class-A bias. This is the recommended bias setting for most small-signal, analog audio amplifiers. Class-A bias allows the maximum signal swing before distortion.
How the JFET amplifies. The varying gate signal causes the JFET's channel resistance (or width) to vary. For this reason, more or less current passes through the JFET. The drain resistor R2 converts the current variations to voltage variations. Since the input signal controls the channel width, we've achieved valve-like operation. That is, a small signal controls a large signal. In our case, the Q1 gate voltage controls the Q1 drain current. This valve-like behavior is the basis of amplification.
The output signal appears between the drain and ground. Capacitor C1 blocks the DC control voltages in the circuit, but passes the amplified AC signal. Besides being amplified, this signal is also an inverted form of the input signal. That is, when the input AC signal goes positive, the output AC signal goes negative. Inversion takes place for this reason: Remember that the input signal comes in across the gate and ground. The gate is more negative than the ground terminal. Now the output comes out across the drain and ground. But we've connected the drain to Vdd. Then the drain is more positive than the ground terminal. With the gate negative and the drain positive, the output signal must flip upside-down. This output signal exits the amplifier through capacitor C1 and appears across resistor R4. C1 blocks DC and passes AC.
External load. Resistor R4 is an optional, external load resistor. The purpose of this resistor is to keep the no-signal output of your amplifier from floating high. This unfortunate effect would only occur if the amplifier connected to a very high impedance without any resistance to ground. Although such connection is unlikely, it might occur. Of course, no advantages come without costs. For the signal, the R4 resistance shunts R2. That is, R4 is an extra burden for the JFET. For this reason, we keep R4 at least 10 times the size of resistor R2. A smaller R4 would seriously reduce the amplifier gain.
Transistor Sockets vs. Zeners
Some builders like to use a transistor socket for the FET. Obviously these builders expect the FET to blow up. And they're right. If you don't protect a JFET from voltage transients, eventually you'll blow it. All you need to do is plug in your guitar on a dry day. One spark destroys the JFET. Yet I don't use transistor sockets, and don't have to. Why? Because zener diodes protect my JFET from most static discharges.
Now let's consider sockets. A socket adds complexity, capacitance and several failure points. You can connect a JFET with three connections. A socketed JFET requires six connections. And three of those are loose, friction connections. Also, the JFET is there to combat the high-frequency rolloff effects of line capacitance. So why add capacitance to the circuit? Use the zeners. Forget the socket.
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