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| The Basic Differential Amplifier | Differential Amplifier with Constant Emitter Current | Differential Amplifier with Active Collector Loads |
| Differential Amplifier with Current Mirror Load | Differential Amplifier with Lee Load | Differential Amplifier Applications |

Some Applications for Differential Amplifiers

Ways to Use the Differential Amplifier

Differential amplifier with constant emitter current

When we think of a differential amplifier, we typically think of a circuit like the one shown to the left. We might replace the collector load resistors with a current mirror, but the emitter current mirror is almost certain to be included. The implication, of course, is that the combined emitter current for Q1 and Q2 should remain constant. This improves common mode rejection, helping to ensure that only input differences to Q1 and Q2 will be amplified and passed on to the next circuit. Of course, there are many cases where this is excactly what we want. This is a primary requirement for operational amplifiers, for example.

On other pages, we mentioned filtering out noise signals that get picked up by long wires attached to a sensing device such as a strain gauge, pressure sensor, or temperature sensor. In such cases, it is generally necessary to have the sensor located away from the circuit it is connected to. After all, we wouldn't want to risk the circuitry in a corrosive or risky environment. Far better to protect just the sensing device and use a standardized circuit to monitor the device.

However, there remains another possibility: suppose that instead of keeping the emitter current constant, we use a data signal of some sort to control it. How many ways could we then use this kind of circuit?

Volume Control

A potentiometer used as volume control. An electronic volume control

The figure shown to the left is the basic volume control. An audio signal is applied to one end of a potentiometer, whose other end is grounded. A mechanical slider is then allowed to move anywhere along the length of the potentiometer's resistance, creating an adjustable voltage divider. The output audio signal is a copy of the input signal, at reduced amplitude. A stereo audio system will have two of these, one for each audio channel, mechanically connected so they are adjusted concurrently.

While this is a very simple, direct, and inexpensive way to adjust audio volume (or any signal amplitude), it has a few problems. Since the movable contact must slide directly on the resistance material, it will cause some wear. Even worse is that the exposed contact can readily collect dirt or dust, causing the contact to be intermittent and noisy.

Now consider the circuit to the right. The audio signal is now providing the reference for the current mirror. This means that the emitters of Q1 and Q2 now receive current that matches the audio signal. However, only the emitter current of Q2 actually reaches the output terminal; any current through Q1 is lost.

The control voltage can vary from -VBE for full volume to +VBE for zero signal level. Most commonly it comes from a small digital to analog converter driven by a binary counter that can count up or down on signal. This is how remote control volume works on modern television receivers, for example.

I-F amplifier with AGC.

Automatic Gain Control (AGC)

The circuit to the right shows the last stage of an intermediate-frequency (i-f) amplifier. Circuits of this type are standard for radio receivers. The stages are transformer-coupled, with the transformer windings tuned specifically to either 455 kHz (AM broadcast band) or 10.7 MHz (FM broadcast band). The diode, capacitor, and resistor at the top are connected only to the final stage; these components sample the average amplitude of the modulated signal being amplified by the overall circuit.

The typical i-f amplifier consists of three such stages. The job of the i-f amplifier is to amplify the signal to a level where the audio information can be extracted from the carrier signal. However, there is a problem here. A nearby radio transmitter will send a very strong signal to the receiver, while a more distant transmitter will have its signal weakened by distance. Nevertheless, we want the signal output from this circuit to always have nearly the same amplitude. This will enable the radio receiver to sound just as loud for weaker stations as it does for stronger ones.

It is the Automatic Gain Control (AGC) circuit that accomplishes this. For weak signals, the AGC circuit at the top has very little output, so Q3 has little effect on the circuit. Therefore the input signal, as amplified by Q1, is passed through Q2 to the output transformer. For stronger signals, the AGC output voltage increases, and Q3 begins to shunt some of the amplified signal away from Q2. An overwhelmingly strong signal will cause Q3 to shunt almost the entire signal away from Q2, drastically reducing the effective gain of the overall stage and the i-f amplifier as a whole.

Of course, it is quite possible to use a single transistor in each stage of the i-f amplifier, and some inexpensive receivers are still built that way. The risk is that then the AGC signal can only operate by changing the dc bias on the transistor. This changes the operating point of each transistor to reduce its efficiency as an amplifier, and may push it far enough to introduce distortion of the signal. By using this approach, we can keep all transistors operating in their linear ranges, keeping the signal as clean as possible.

Amplitude modulation.

Amplitude Modulation

If we remove the AGC detector from the circuit above, we can apply a different signal to Q3. Consider what will happen if we apply a radio frequency signal to the input transformer, and an audio frequency signal to Q3. This gives us the circuit shown to the right.

Here, the input signal to Q1 is a radio frequency (rf) signal at constant amplitude, such as the signal generated by an appropriate oscillator. This signal is amplified by Q1 and fed up to Q2 and Q3. The audio signal applied to Q3 causes the effective gain of Q2 to change at that frequency. The result is an amplitude modulated rf carrier at the output. This could easily be the signal generated by a commercial AM broadcast radio station.

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