This circuit can be used as a preamplifier for microphones or as an amplification stage for headphones in experimental receivers (galena). The circuit is simple to assemble and has very low power consumption.
This circuit has a very small consumption, so that the battery that feeds it will have a very great durability.
The input impedance is very high and the output impedance is low, making it possible to connect to different types of headphones or to the input of amplifiers.
In figure 1 we have the complete diagram of the preamplifier.
As it is very simple circuit, its assembly can be made based on a small terminal strip, as shown in figure 2.
On assembly, observe the position of the transistor and the polarity of the battery, because with inversions the circuit does not work.
The transistor may also be the MPF102, but it has terminal arrangement different from the BF245.
Material list
Q1 - BF245 - field effect transistor
S1 – On-Off switch
B1 - 9 V - battery
R1 - 2M2 x 1/8 W - resistor - red, red, green
R2- 1k ohms x 1/8 W- resistor - brown, black, red
R3 - 4k7 ohms x 1/8 W - resistor - yellow, violet, red
C1 - 220 nF - ceramic or polyester capacitor
C2 - 47 uF - electrolytic capacitor
C3 - 22 uF - electrolytic capacitor
C4 - 100 uF - electrolytic capacitor
Several:
Terminal strip, battery connector, input and output jacks, wires, solder, etc.
