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Transistor Audio Amplifier Circuit Simple Fixed Operating Bias Guitar Pre-Amplifier

OddMix.com - Transistor Technology Note - TN090927 - Karl Nagy

Figure 1. Fixed Bias Transistor Audio Amplifier [4 KB]
Fig. 1. Fixed Bias 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 this simple, small signal, common emitter, NPN transistor amplifier circuit the input signal is a 1 kHz, 10 mV AC voltage. Usually microphones or turntable cartridges have similarly small signal levels that they require a pre-amp circuit. Our simple amplifier can be used as a microphone, phonograph or guitar pre-amplifier to amplify their low-level signals. Attaching the same amplifier to the output of a crystal detector radio would also result in noticeable improvement in the set's output audio volume.

Figure 2. Oscilloscope Trace of Fixed Bias Transistor Audio Amplifier [4 KB]
Fig. 2. Oscilloscope Trace of Fixed Bias Transistor Audio Amplifier

This amplifier outputs a 1.2-Volt signal as shown by the red trace on the oscilloscope display. 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 - Figure 2. Since the gain of an amplifier is the ratio of the output to the input signal this amplifier stage has an AC gain of 1,200 mV/10 mV = 120. Since the forward transfer ratio hFE (DC Beta) or gain is different for each 2N3904 transistor, the amplifier gain may vary from below 100 to 300 or more.

In spite of this respectable gain the self biased amplifier circuits largest drawback is the total lack of protection for the transistor. Any increased supply voltage result in a corresponding increase of the collector and bias currents. An increase of the bias current always results in an increased collector current. It is easy to realize that any increase of the supply voltage has a compounded adverse effect of increasing the transistor collector current.

Because of that, this type of fixed bias amplifier circuit does not protects the transistor against self distract by thermal runaway. In spite of that, because of its minimal parts requirement and inherent simplicity, for small signal applications this biasing method is used often. In real circuits, R1 is usually made of two parts, a fixed resistor in series with an adjustable one. That way the transistor biasing can be made quickly with screwdriver and a simple collector current measurement.

                      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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