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Amplifier Audio Transistor Guitar Pre-Amp Circuit Simplest Operating Self Bias

OddMix.com - Transistor Technology Note - TN090926 - Karl Nagy
Figure 1. Self Biased Transistor Audio Amplifier [3 KB]
Fig. 1. Self Biased Transistor Audio Amplifier

The simplest way to bias a transistor amplifier is by the fixed-bias circuit. The supply voltage is connected to the transistor's base terminal trough the R1 current limiting resistor. The value of the bias resistor is usually selected based on the transistor's gain from its data sheet and the selected operating collector current. Resistor R2 is the load resistor for transistor Q1. The function of this R2 load resistor is to limit the collector current to safe values. The C1 and C2 coupling capacitors are used to separate the AC signals from the transistor's DC bias current. Through these two capacitors, C1 and C2 connect the input and output AC [Alternating Current] signals.

In our simple, small signal, common emitter, transistor amplifier circuit the input signal is a 1 kHz 10 mV AC voltage. Usually microphones or turntable cartridges have similarly small signal levels. Thus our simple amplifier can be used to amplify similar low-level signals. Attaching the same amplifier to the output of a crystal detector radio would result in noticeable improvement in the output audio volume.

Figure 2. Oscilloscope Trace of Self Biased Transistor Audio Amplifier [4 KB]
Fig. 2. Oscilloscope Trace of Self Biased Transistor Audio Amplifier

This amplifier outputs a 0.8-Volt signal as shown by the red trace on the oscilloscope display - Figure 2. Notice that the output signal is not completely symmetrical, but the minor asymmetry does not affect the circuit operation. The scale for the blue trace is 10 mV per division and for the red trace it is 1 V per division. Since the gain of an amplifier is the ratio of the output to the input signal this amplifier stage has an AC gain of 800 mV/10 mV = 80. Since the forward transfer ratio hFE (DC Beta) or gain is different for each 2N3904 transistor, the amplifier gain may vary from below 50 to 300 or more.

In spite of this respectable gain the self biased amplifier circuits largest drawback is reduced gain and input impedance. Because the bias resistor R1 of this amplifier is connected to the R2 load resistor there is a negative feedback that works to reduce the amplification. Because of this negative feedback this type of amplifier circuit protects the transistor against self distract by thermal runaway. As the collector current increases, the bias current decreases and that forces a reduction in the transistor's conduction.

                      Parts list for this one transistor audio amplifier:

                         C1, C2 - Electrolytic Capacitor 10 uF 6-10 VDC
                         R1 - Resistor, 200 K, 1/4 Watt Carbon
                         R2 - Resistor, 1 K, 1/4 Watt Carbon
                         Q1 - Transistor 2N3904, NPN, silicon, amplifier
                         B1 - Battery - 6 Volt, 4 AA cells

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