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Project Notes

#495 Negative Feedback Amplifier

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

Build

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

Schematic

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

Breadboard Construction

Breadboard

NegativeFeedbackAmplifier_bb_build

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

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

scope_bb

Protobaord Construction

protoboard_layout_and_board

Under test:

under_test

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

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

scope_out

Credits and References

About LEAP#495 BJT
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.