Differential amplifier
A differential amplifier is an electronic amplifier with two inputs, in which not a single signal, but the difference of the two input signals U1 and U2 is increased. It is used in operational amplifiers, comparators, as a DC voltage amplifier for measuring purposes, the balanced signal transmission, but also in some oscillators.
Operation
A voltage difference between the inputs leads Ua with ideal differential amplifier to the output signal with
Where V is the gain factor. If the same voltage at the two inputs, the output signal is ideally zero. The output voltage depends on ideal differential amplifiers with optimal " common mode rejection " ( CMRR ) only by the voltage difference, but not the absolute height of the two voltages Vi1 and Vi2 from, as long as both do not exceed the scope of the operating voltage. Not optimum common-mode rejection, however, causes an equal change in voltage Vi1 and Vi2 by an amount DELTA.U (ie in unison ) a generally slight change in the output voltage Ua.
Internal circuit
The differential amplifier consists of two identical transistors Q1 and Q2 have a common emitter resistor Re. To improve the common mode rejection of these normally consist of a constant current source, implemented in the image with the transistor Q3. In the example about 1.3 mA to flow through Q3.
The current through Re is the sum of the currents through Q1 and Q2. In an exactly symmetrical structure, the same voltage should be measured and the difference is Ua zero at each of the two collector resistors.
If now, for example, the voltage Vi1 to 2 mV greater than Ue2, so will flow through Q1 more power. Wherein the voltage drop across the resistor Ra1 = 5 k and decreases the voltage at the collector of Q1. Because the sum of the two collector currents but is constant, flows through Q2 correspondingly less power, the voltage at the collector of Q2 is increased. The differential voltage Ua between the two panels is then - depending on the quality of the transistors - for example, 1 V. With these sample values , the gain
In contrast, when both input voltages change in the same direction, the voltages at the collectors do not change, since the currents through Q1 and Q2 can not change in the same direction, otherwise their sum would no longer be constant.
The dynamic range of the circuit is less than the operating voltage, because each transistor has a certain residual voltage of about 0.6 V is required in order to function properly. In the circuit shown, the input voltages shall be ( of the zero volt rail from measured ) between about -2 V and 1.5 V.
The common-mode rejection is highly dependent on the symmetry of the circuit, a different temperature of the two transistors Q1 and Q2 can already deteriorated. To build differential amplifier therefore today as an integrated circuit because the entire assembly is then seated on a common chip, and is therefore maintained at a uniform temperature.
The picture to the differential amplifier having an output stage (M5 ) is extended to obtain an operational amplifier. This consists of
- The differential amplifier M1/M2 at Uin input
- The current mirror M3/M4, which replaces the two load resistors, whereby the gain of the differential amplifier increases significantly. In addition, the balanced output of the differential amplifier is made asymmetric.
- A kept very simple, single-ended output stage M5.
Instead of resistors, the two constant current sources I1 and I2 are used. All these measures and the consistent use of field -effect transistors are used to increase the dynamic range.
This simple and much used in integrated circuit technology is circuit Miller operational amplifier, because the Miller effect compensation for the frequency, that is, to suppress the tendency to oscillate at high frequencies is employed. The total gain at low frequencies is more than 105, and decreases due to the Miller capacitor with increasing frequency.