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Stabilisation of the operating point

Stabilisation of the operating point

Item no.: P1378600

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Negative feedback is used to increase the stability of a transistor amplifier stage against fluctuations in temperature, operating voltage, and scattering of characteristic amplification quantities. This is achieved by feeding back a portion of the output voltage to the input of the amplifier with reverse phase position. For the negative current feedback used in this experiment, the amplified current in the emitter resistor produces the reverse-phase voltage (proportional to the current) used for the negative feedback. In a negative voltage feedback, however, a portion of the amplified voltage from the collector is fed back to the base of the transistor.
Aside from the desired effects (increasing the stability of the amplifier, linearisation of its characteristic curve, and modification of its characteristic quantities), negative feedback is always accompanied by a decrease in amplification. The larger the ratio of feedback voltage to output voltage, the larger the decrease in amplification.
The stabilising effect of negative current feedback can be understood, assuming, there is a constant control voltage of, for example, 1 V at the output of the amplifier. This is divided between the base emitter line and the emitter resistor. Now, if the emitter current increases due to an increase in temperature or operating voltage, for example, then the voltage at the emitter resistor also increases. Consequently, only a small portion of the connected control voltage is effective for the scattering of the transistor for the base-emitter line. This counteracts the original increase in collector current. The same thing happens when an alternating voltage is used for controlling. Only a portion of the connected voltage is available for controlling the transistor when an emitter resistor is in the circuit.
If a capacitor with a proper capacity is connected in parallel to the emitter resistor, then the negative feedback is canceled in the case of alternating voltage. This is not true for negative direct current feedback, though.


  • No additional cable connections between the building blocks needed - clear arragned and quick setup
  • Contact saftey due to puzzle blocks system
  • Corrosion-free gold plated contacts
  • Doubled earning sucess: Electric circuit diagram on top, real components can be seen unterside


How can a transistor amplifier stage be made insensitive to fluctuations in operating voltage?

Investigate how a transistor amplifier stage reacts when an emitter resistor is added to the circuit.


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