A JFET preamp for beginners. Simple yet elegant. Low noise.
The following is an example for a very simple and high quality JFET preamp. You can use this to amplify a very low signal sources. FET transistors are usually not used independently in preamplifiers. In most of the cases they are paired with bipolar transistors.
R2 automatically sets the bias for Q1 in a way similar to a tube circuit. This resistor tides the gate of Q1 to ground and sets its potential. Source’s potential is higher than ground by the voltage across R4 and R5. In other words the gate is more negative then the source. The gate-source transition represents a normal PN junction. Usually this junction is tied in opposite direction, which explains the relatively high input impedance (more than 109 Ω).
Basically the input impedance of the circuit is set by R2 and in this case it’s around 1MΩ. R4 and R6 form an AC negative feedback. R1 and R7 serve as over voltage limiters and over current limiters in case of short circuit at the input or output.
Besides their high input impedance, the FET transistor preamps have one more advantage. This is the very low noise. This means that we can easily design a stage with high S/N ratio.
The following picture shows the DC operating points.
Considerable deviations from these values will cause the stage not to perform as it should be. 0V at the collector of Q2 means that there is a break into collector-emitter junction while 12V means there is a short between those two outputs. The voltage at the base of Q2 is equal to one voltage drop of a normal junction which is about
If the DC operating points are not showing any deviations but we don’t have enough gain then the suspect is the C1 capacitor. Q1 works is worst conditions then Q2. If it can’t be driven properly this means the bias is not good. A voltage too high on its source will indicate this.
The circuit is limited to only few volts of output swing. That’s why its suitable for low voltage applications like phono cartridge preamps etc.