Transistor amplifier
A transistor amplifier is an electronic circuit in which a small input signal controls a transistor which, due to its amplifying properties, emits a larger output signal (larger output current and / or larger output voltage).
Task and function
An amplifier circuit is used to amplify electronic signals. To do this, it needs an "active component". In the case of the transistor amplifier, this can be an NPN or PNP transistor (the polarity of the operating voltage depends on this). The material used to manufacture the transistor ( germanium or silicon , more rarely gallium arsenide ) has no fundamental importance for the function of the circuit. However, it can have effects on temperature stability, operating point setting, etc. In addition to the active component, passive components are required:
- Resistors for setting the operating point
- Capacitors for DC voltage decoupling (coupling capacitors)
Basic circuits
A transistor can be operated in three basic circuits :
- Emitter circuit - the emitter is a common electrode for input and output signals
- Collector circuit - the collector is the common electrode for input and output signals
- Basic circuit : The basis is a common electrode for input and output signals
parameter | Emitter circuit | Collector circuit | Basic circuit |
---|---|---|---|
Current amplification | 10-100 | 10-250 | <1 |
Voltage amplification | 100-1,000 | <1 | 100-500 |
Power amplification | 1,000-100,000 | 10-250 | 100-500 |
Phase shift | 180 ° | 0 ° | 0 ° |
Input resistance | 0.4 Ω - 5 kΩ | 200 Ω-500 kΩ | 50 Ω-500 Ω |
Output resistance | 10 kΩ-100 kΩ | 1 kΩ – 10 kΩ (depending on the wiring) |
0.1 Ω-2 MΩ |
Bandwidth | 20 Hz-200 Hz | ||
Specialty | good suitability as an impedance converter | good high frequency properties |
Example for emitter circuit
In the transistor amplifier shown here, the input signal is fed to the base of the transistor via a coupling capacitor 10 µF. It controls a stronger current in the output circuit (collector-emitter circuit). Together with the 4.7 kΩ load resistor, the transistor forms a voltage divider . The higher the base voltage of the transistor, the lower its resistance and thus the lower the output voltage (across the transistor). This leads to a phase reversal between input signal U e and output signal U a . The output signal is passed on to the next module via the coupling capacitor on the right.
The voltage divider from 47 kΩ and 10 kΩ sets the operating point of the transistor by means of a DC voltage applied to the base, which is obtained from the operating voltage . As high-pass filters , the coupling capacitors ensure that only the signal (alternating voltage) is transmitted, but no direct voltage, because this would change the operating point. A more detailed description of the negative feedback is given in the article Amplifier (electrical engineering) .
Special circuits
There is a wide range of specialty circuits.
- Push-pull circuit : The push-pull circuit is one of the most important special circuits. It is made up of at least two transistors that work together to achieve greater output power. Both transistors are controlled in phase opposition. This allows non-linearities of the characteristics to be compensated. The operating points are often set in such a way that each of the two transistors only amplifies one half-wave.
- Directly coupled amplifier stages : Often the amplification of a single amplifier stage is not sufficient. For this reason, several stages are combined; In the case of transistors, several transistors of different conductivity (PNP and NPN) are often connected.