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Amplifier Audio Transistor Circuit Voltage Divider Bias Guitar Phono Pre-Amplifier

OddMix.com - Transistor Technology Note - TN090928 - Karl Nagy

Figure 1. Voltage Divider Bias Transistor Audio Amplifier [5 KB]
Figure 1. Voltage Divider Bias Transistor Audio Amplifier

This voltage divider method is how most transistor amplifier circuits are biased. This circuit gives excellent protection for the transistor while it allows proper amplifier stability and selections for circuit gain and input impedance.

The B1 supply voltage is connected the voltage divider formed by resistors R1 and R2. The transistor's base is connected to the junction of resistors R1 and R2. Resistor R3 is the collector load resistor for transistor Q1 and R4 is the emitter resistor. The values of the R1 and R2 bias resistors are usually selected based the transistor's gain taken from its data sheet and from the selected operating collector current. The values of the R3 load resistor and R4 emitter resistor determine the transistor Q1s collector current and set it to a safe value. 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. Capacitor C3 is used to set AC gain for the amplifier.

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. This simple amplifier can be used as a microphone, phonograph or guitar pre-amplifier to boost 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 Voltage Divider Bias Transistor Audio Amplifier [4 KB]
Fig. 2. Oscilloscope Trace of Voltage Divider Bias Transistor Audio Amplifier

This amplifier outputs a 0.9-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. Since the gain of an amplifier is the ratio of the output to the input signal this amplifier stage has an AC gain of 900 mV/10 mV = 90. 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.

As this somewhat lower gain attest, voltage divider biased amplifier circuits trade some circuit amplification for stability and protection for the transistor. The negative feedback helps to fix the DC and AC gain of this amplifier at the expense of more circuit components and complexity.

                      Parts list for this one transistor audio amplifier:

                         C1, C2 - Electrolytic Capacitor 10 uF 6-10 VDC
                         C3 - Electrolytic Capacitor 100 uF 6-10 VDC
                         R1 - Resistor, 100 K, 1/4 Watt Carbon
                         R2 - Resistor, 30 K, 1/4 Watt Carbon
                         R3 - Resistor, 1 K, 1/4 Watt Carbon
                         R4 - Resistor, 100 Ohm, 1/4 Watt Carbon
                         Q1 - Transistor 2N3904, NPN, silicon, amplifier
                         B1 - Battery - 6 Volt, 4 AA cells

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