If we want a high impedance preamplifier circuit. Because we use a crystal microphone or we use it with a ceramic record player, etc. These devices require the input of a high impedance amplifier This circuit also has a low damping value. Make the sound come out concisely.
Of course, we like choosing simple and cheap circuits, this circuit too.
See the active circuits below. It’s a simple circuit. With only one transistor, original characteristics.
This circuit is one of 4 transistor preamplifier circuits. All circuits have PCB so easy to learn and having fun.
- Low impedance input Preamplifier
- The medium Impedance Preamplifier circuit
- High Impedance Preamplifier circuit
- Simple Preamplifier using FET
You should read all 4 articles for more understanding.
We designed it using the principle of emitter follower circuit (common collector). In this circuit design, makes the input impedance high And low impedance output.
The input impedance of this circuit is equal to the gain (hFE) of the transistor times the resistance of the emitter pin (R2).
How does the circuit work
What is an emitter follower circuit? It is a type of transistor circuit that has an input signal through the base pin (ฺ B). Then amplify the signal to get stronger and go out through the emitter pin (E).
When the bias had finished arranging for this circuit.
We feed the power supply through decoupling consisting of R3, C3 directly into the collector pin (C) of Q1, and feed the bias current to Q1 through R1 as well. Next, the output or load across from R2 which is the emitter resistor.
If the transistor used as a high gain transistor will give the input impedance of approximately 100K.
This circuit has the advantage of extending the high-frequency signal well. Less high-frequency loss. And solve the problem of hum or RF noise. When playing with record players or guitar well.
The output impedance is approximately 1,000 ohms and the output voltage is slightly higher than unity (1 time).
How to build
See the Actual-size of Single-sided Copper PCB layout which has the same characteristics as other Preamplifier circuits. And see the component layout.
Components layout of high impedance preamplifier
Parts you will need
0.25W Resistors, tolerance: 5%
- R1: 1M
- R2: 4.7K
- R3: 100 ohms
- C1: 0.1uF 50V Ceramic
- C2: 10uF 25V Electrolytic
- C3: 100uF 25V Electrolytic
- Q1: BC549C or equivalent, 45V 0.1A, NPN Transistor
How to increase the input impedance
If we want to increase the input impedance of the bipolar circuit even more We can do this by using the positive feedback circuit. From the emitter pin back to the base to build a network circuit for Bias.
The above method, if made easy We have to use 2 transistors, like the amplifier circuit on the tape radio. That is commonly used in the olden days.
But if you still want a high impedance preamplifier circuit that only uses one device. We have to change from transistor to FET. Read more in the article.
Check out these related circuits, too:
- High impedance small amplifier circuits
- Video amplifier splitter circuit using transistor
- Let’s try the 3 transistors Audio Amplifier circuits
- 4 Preamplifier circuits using transistors
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