Medium Impedance Preamplifier circuit using transistor

If we want to design a Medium Impedance Preamplifier circuit using only one transistor We should change it into a common emitter.

This circuit has an input impedance of about 1.5K and an output impedance of about 4.7K. We can think from the load at the output. It is the value of R2 which is 4.7K.

Medium Impedance Preamplifier circuit using transistor
Note: This circuit is one of 4 transistor preamplifier circuits. All circuits have PCB so easy to learn and having fun.
  1. Low impedance input Preamplifier
  2. The medium Impedance Preamplifier circuit
  3. High Impedance Preamplifier circuit
  4. Simple Preamplifier using FET
You should read all 4 articles for more understanding.

Circuit details

Look at the circuit diagram.

Medium Impedance Preamplifier circuit using transistor

How it works

When considering the arrangement of the circuits as shown in the circuit diagram, it is found that the power supply is 9 volts. It will flow through the decoupling devices consisting of R3, C4 before.

Then, it supplies the positive voltage to the lead collector (C) of Q1, with R2 as a load of this part.

At the pin base (B) of the transistor requires a positive voltage when compared with the pin emitter(E). But it is positive lower than colletor just a little bit.

So, We use an easy way that takes R1 pass the voltage from pin collector into the base.

We called this that self-bias or negative feedback bias. Which it is a method of bias that can be varied according to the power supply of the circuit.

The emitter pin uses VR1 to act as a bias. With C2 to bypass the gain ratio. We designed this circuit can change the rate of extension. By moving the bypass point to increase or decrease the gain of the circuit according to the input from the audio source.

We are able to use other transistor numbers – they need to have a sufficiently wide hFE gain of about 10 to 160 times.

How to build

We can assemble this circuit on universal PCB or make an Actual-size of Single-sided Copper PCB as the layout below.

Copper PCB layout Medium Impedance Preamplifier
Copper PCB layout Medium Impedance Preamplifier
Components layout of the medium impedance preamplifier circuit

Parts you will need

0.25W Resistors, tolerance: 5%

  • R1: 1M
  • R2: 4.7K
  • VR1: 470 ohms


  • C1, C3: 4.7uF 25V Electrolytic
  • C2: 47uF 25V Electrolytic
  • C4: 100uF 25V Electrolytic


  • Q1: BC549  45V 0.1A, NPN Transistor

Basic Adjustment

When we have all the equipment And all the equipment has been installed, let’s test.

If we adjust VR1 go to the voltage 0V. It will create a voltage gain of about 8 to 10 times. When the hFE of a transistor is 110 to 600 times.

If we adjust VR1 go to pin emitter, the voltage gain will be 80 to 100 times.

The current of the 9V power supply is not less than 1.25mA.

Read Also:

Hum and noise Protection

The input of this circuit is designed to add the capacitor. See in the PCB. To solve the problem of RF frequency interference, too.

Adding capacitor to reduce hum RF noise

By connecting the input point (connecting the input signal cable). Put the capacitor into the hole of the PCB. Which we designed, It is approximately 0.001uF to 0.01uF.

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