Differential amplifier

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The differential amplifier is an electronic amplifier with two inputs in which the difference between the two input signals is amplified. Input signals that have the same effect on both inputs are ideally not amplified. This amplifier circuit is used as an essential basic circuit in the input stage in operational amplifiers and comparators , also as a DC voltage amplifier for measurement purposes, in the field of symmetrical signal transmission and in some oscillator circuits .

functionality

Differential amplifiers are usually designed in such a way that electrical voltages are used. A voltage difference between the two inputs with the voltages U e1 and U e2 leads to the output voltage U a in a differential amplifier assumed to be ideal , expressed as an equation:

with the differential gain factor V ds . If the same voltage is applied to both inputs, the output signal is ideally zero. In the case of a differential amplifier that corresponds more to reality with a not infinitely high common mode rejection (CMRR), there is a proportion due to the common mode gain:

with the common mode gain factor V c . With a common mode gain factor different from zero, an equally large voltage change in U e1 and U e2 causes a generally undesirable additional change in the output voltage U a .

Realizations

Bipolar transistor

Simplified basic circuit with bipolar transistors

A differential amplifier built up with bipolar transistors consists, as in the figure opposite, of two similar transistors Q1 and Q2, with a common current source at the two emitters. To improve the common mode rejection, this current source is normally designed as a constant current source with the current I 0 . The attached at the top of the current mirror ( English current mirror ) results in a single-ended output to be connected externally through a load resistor must, an output current I a to let it flow. In contrast to ohmic collector resistors, a current mirror offers the advantage of a larger differential gain factor.

Transfer characteristic for differential amplifiers made of bipolar transistors (I 0 = I ABC , I a = I out, U e1 = U in1, U e2 = U in2 )

With an exactly symmetrical structure and a differential input voltage of zero, the current at the output is ideally zero. If the differential voltage is different from zero, the output current is a good approximation

with the temperature voltage U T a. If both input voltages U e1 and U e2 change in the same direction, the output current does not change. After linearization , the formula is simplified as follows:

The common mode rejection depends heavily on the symmetry of the circuit, a different temperature of the two transistors Q1 and Q2 can worsen it. Therefore, differential amplifiers are commonly referred to as integrated circuit constructed as then the complete circuit on a common The can be operated at approximately the same temperature. The control range of the circuit is smaller than the operating voltage because each transistor requires a certain residual voltage of about 0.6 V in order to function correctly.

Field effect transistor

Miller operational amplifiers with MOSFETs

A differential amplifier can be constructed similarly with field effect transistors or electron tubes. In the picture opposite, the differential amplifier is expanded with an output stage (M5), resulting in an operational amplifier. This consists of

  • the differential amplifier M1 / ​​M2 at the U in input
  • the current mirror M3 / M4, which also makes the output of the differential amplifier unbalanced.
  • a very simple, unbalanced output stage M5.

The two constant current sources I1 and I2 are used instead of resistors. All of these measures and the use of field effect transistors serve to enlarge the dynamic range.

This simple circuit, which is widely used in integrated circuit technology, is called Miller operational amplifiers, because the Miller effect is used for frequency compensation , i.e. for suppressing the tendency to oscillate at high frequencies. The total gain is more than 10 5 at low frequencies and decreases with increasing frequency because of the Miller capacitor.

See also

literature

  • Manfred Seifart: Analog circuits . 3. Edition. VEB Verlag Technik Berlin, 1989, ISBN 3-341-00740-7 , Chapter 4: Differential amplifier.

Web links