Differential amplifier amplifies the difference between two voltages and . Differential amplification has many applications, such as the first stage of operational amplifiers (Op-amps).
The two transistors and in the circuit are identical with the same properties, and their emitters are connected to a current source with constant current so that . If increases, will decrease, and vice versa. Consider these three cases:
A simple current source is also shown in the figure. The base voltage of the transistor is fixed at approximately , so that the load current is also approximately constant, independent of the load, i.e., the circuit can be used as a current source providing a current determined by but independent of the load. A better way to hold constant is to replace the diodes by a reverse biased Zener diode. When a zener diode is reversely biased by a voltage exceeding its breakdown voltage, the voltage drop across it, in the circuit, is held at the breakdown voltage, a constant value independent of any other variables in the circuit. Consequently is also constant.
The current mirror circuit shown below is a simple current source that provides a constant current independent of the load .
This circuit is composed of two matching transistors and with identical behaviors such as the input and output characteristics and . They are the input and output stages of the circuit, respectively. As the input, the reference current can be determined as
Again, here transistor can be considered as a current-voltage converter by which the current through is converted to the base voltage shared by both and . The following negative feedback hold the load current constant:
Darlington transistor (Darlington pair) is a compound
structure composed of two transistors, of which the emitter
current of the first transistor becomes the base current of
of the second transistor. The main advantage of the Darlington
transistor is its high current gain
can be found by the following steps:
By properly settng the DC operating point of the transistor circuit, it can be working in any one of the following modes:
This circuit can be considered as a class AB amplifier that is typically used as the last stage of an amplification system, such as in an op-amp circuit, for power amplification with large current and low output resistance to drive a heavy load (small ). A push-pull circuit is composed of a pair of two transistors that work in alternation during the two half cycles of the sinusoidal signal. The circuit can be implemented in either of the following two ways:
An oscillator is a circuit that receives no input but generates a sinusoidal output at a desired frequency. A typical oscillator circuit is based on an active component (a transistor or an op-amp) with positive feedback and an LC circuit (tank circuit). Initially trigged by switching on the circuit, the LC circuit starts to resonate at frequency , and the active component with positive feedback compensates for the attenuation due to the inevitable resistance in the circuit and keeps the oscillation going.
Specifically, the Hartley and Colpitts oscillators are two typical oscillation circuits. In either cases, a transistor amplifier is used to receive positive feedback taken from the LC circuit as a collector impedance , which is maximized at the resonant frequency, thereby the voltage gain of this circuit is also maximized. A fraction of the sinusoidal at the collector is positively fed back to the emitter to prevent attenuation.
When a transistor is used for amplification, its DC operating point of a type A amplifier is typically set in the middle of the load line to maximize the linear dynamic range. By so doing, the signal distortion will be minimized by avoiding the nonlinear region of the transistor circuit.
However, in some applications, the nonlinear behavior of the transistor circuit is taken advantage of, such as in a frequency mixer, used for converting all radio frequencies of different radio/TV broaccast channels to an intermediate frequency , so that the amplification circuit of the receiver can be specialized for this intermediate frequency, instead of a wide range of all possible broadcast frequencies. In radio reception, KHz for AM (535-1605 KHz) and MHz for FM (88-108 MHz).
As discussed previously, the output current is approximately an exponential function of the input voltage :
Note that the specific nonlinear behavior of the circuit is not important, as the Taylor series expansion of any nonlinear function will contain constant, first and second order terms as the exponential function assumed above, and the same frequency components will result. Frequency mixer is an important component in super-heterodyne reception which is used in all modern radio and TV broadcasting. Here the frequency of the local oscillator is changed by a variable capacitor, which can be adjusted jointly with the capacitor of the tuning circuit, so that the of the local oscillator changes with the carrier frequency (radio frequency) of the broadcast signal received by the antenna in such a way that their difference is always a constant:
The circuit diagram of a simple super-heterodyne radio receiver is shown below. Note that the first transistor is an oscillator that also receives signal from the LC tuning circuit at the base, i.e., it is also a mixer that mixes two frequencies. The next two transistors amplify frequency component the signal from the mixer, but