Multi-Stage Transistor Amplifiers

The following amplifier circuit has a problem. Despite the presence of a strong input signal (as verified by an oscilloscope measurement at TP1), there is no sound coming from the speaker:

Explain a logical, step-by-step approach to identifying the source of the problem, by taking voltage signal measurements. Remember, the more efficient your troubleshooting technique is (the fewer measurements taken), the better!

A common wideband transistor amplifier circuit is the cascode design, using common-emitter and common-base transistor stages:

What advantage(s) does the cascode amplifier have over “normal” single- or multi-stage amplifier designs? What, specifically, makes it well suited for high-frequency applications, such as RF (Radio Frequency) signal amplifiers?

Suppose the following three-stage transistor amplifier were constructed:

With no emitter swamping resistors anywhere in this circuit, the voltage gain of each stage is guaranteed to be large, but unstable as well. With three stages arranged like this, one feeding into the next, the final voltage gain will be very large, and very unstable.

However, if we add another resistor to the circuit (R_feedback), something very interesting takes place. Suddenly, the amplifier circuit’s overall voltage gain is decreased, but the stability of this gain becomes much improved:

Interestingly, the voltage gain of such a circuit will be nearly equal to the quotient of the two highlighted resistors, \(R_{feedback}\) and \(R_{in}\):

$$A_V \approx \frac{R_{feedback}}{R_{in}}$$

This approximation holds true for large variations in individual transistor gain (β) as well as temperature and other factors which would normally wreak havoc in the circuit with no feedback resistor in place.

Describe what role the feedback resistor plays in this circuit, and explain how the addition of negative feedback is an overall benefit to this circuit’s performance. Also, explain how you can tell this feedback is negative in nature (“degenerative”).