#495 Negative Feedback Amplifier

Examining the design and performance of an ac-coupled negative feedback amplifier constructed with BJTs.

Notes

Negative feedback is by definition the concept of using a fraction of the output to dampen the input. When designed appropriately, this can trade some gain for higher linearity and stability, and reduced sensitivity to component parameter variations.

Circuit

The circuit design used here is based on a fairly common “Negative Feedback Amplifier Kit” (easily found on Aliexpress or eBay). It essentially a two-stage common-emitter amplifier with a feedback loop from the output of the final stage to the emitter of the first stage. NB: the circuit is exactly as covered in 10-5 Two-Stage Feedback of Electronic Principles by Albert Paul Malvino.

The feedback loop creates an AC voltage divider between the output and ground:

ve = RF1/(RF1 + RF2) . vo

Thus the negative feedback: the first stage output is reduced in proportion to an increase in the second stage output

See LEAP#389 TwoStageCommonEmitterAmplifier, for a similar two-stage CE amplifier but without feedback.

Breadboard Construction

Scope trace with a 1kHz 200mV peak-peak input and finely adjusted bias and feedback:

CH1 (Yellow): input CH2 (Blue): output

Protoboard Construction

Under test:

Scope trace with a 1kHz 200mV peak-peak input and finely adjusted bias and feedback:

CH1 (Yellow): input CH2 (Blue): output

Credits and References

• Negative-feedback amplifier - wikipedia
• Chapter 4 - Bipolar Junction Transistors - Feedback
• 10.1 Cascade of two single transistor stages - Analog wiki
• Design of 2 stage BJT CE amplifier
• Electronic Principles by Albert Paul Malvino
• Multistage Transistor Amplifiers
• LEAP#389 TwoStageCommonEmitterAmplifier