Now lets look at how to use JFETs for small signal circuitry.
The three main differences between JFETs and BJTs are:
- Almost no current flows into a JFET gate, so we don't have to worry about current gain.
- BJTs require a forward voltage of about 0.7V between base and emitter to switch them on. JFETs conduct a high current with zero voltage between gate and source. They are normally run at a lower current, with a reverse bias voltage between gate and source.
- The transconductance of JFETs is generally much lower than BJTs, running at the same current.
The first two can make biasing very easy and allow us to build very simple circuits. For example, this picture shows a simple common source voltage amplifier and a unity gain buffer.
Choosing the parts can be difficult, though. For example, if we want no DC at the output of the buffer, we must either make sure that the two JFETs have exactly the same Vgs, or we must use different values for the source resistors.
However there is a problem: We cannot tell from the datasheet what the Vgs of the JFETs will be, it can vary over a wide range. So for the first option, if we want two JFETs with the same gate-source voltage, we will have to buy more JFETs than we need, measure each one, and choose two that match.
For the second option, we only need to buy two JFETs but we will still need to measure them before we can decide what value resistors to use.
Looking at the buffer circuit again, we can see that there is a much easier way. If we just add a coupling capacitor at the output, there is no problem with DC, and we don't need to measure any JFETs.
This is an important lesson. When we are working with JFETs, we will often find ourselves having to measure, match and select individual JFETs unless we can design our circuits in such a way that big differences in JFET parameters do not have much effect.
This is not always as easy as it sounds. What if we want to use a differential pair (also called a long tailed pair or LTP) for the input stage of a power amplifier? How do we make sure there is no DC voltage across the loudspeaker?
We could put a big coupling capacitor at the output of the amplifier, but there may be good reasons why we don't want to do that. In that case, it looks like we really so need carefully matched JFETs for the input stage, unless we can think of some other trick to get rid of the DC.
In this case, the easy way is to use a trimmer potentiometer to adjust the DC voltage at the output to zero. This is a good solution but we must be careful how we use the trimmer in the circuit. Remember that trimmers (and other potentiometers) are not very reliable. If the wiper becomes disconnected we do not want to have the whole power supply voltage across the loudspeaker as this will destroy it. We must design the circuit in such a way that there will be no more than a couple of volts across the speaker if the trimmer fails.
If you look back at the MOSFET output stage I showed earlier, you will see a similar precaution. In that circuit, if the trimmer fails open circuit, bias is removed from the MOSFETs. There will be some distortion of the audio but nothing will be damaged.
If we swap the position of R5 and the trimmer, we can still adjust the bias to the correct value, but it will be a very bad design. If the trimmer fails, the MOSFETs will be overbiased, very high current will flow through them, and they will probably be destroyed.