The differential amplifier (DA) is used to amplify the difference between two input signals. It can be considered as an analog circuit, consisting of two inputs and one output.
Amplifiers used in various electricaland electronic circuits for generating signals and performing mathematical operations, are called operational amplifiers (op amps). They are the key components of an electronic analog computer. Their invention in the early 1940s led to the replacement of mechanical counting devices with quiet and fast electronics. Many analog computers relied on vacuum tubes, available from George Philbrick in 1952.
In 1963, Bob Vidlar at Fairchild Semiconductor made the op amp on a single integrated circuit A702 - the very first monolithic operational amplifier IC.
Transistor Amplifier Circuit
The differential operational amplifier can be assembled according to the scheme, as shown in the figure below, which consists of two transistors T1 and T2.
The remote control circuit has two inputs I1 and I2 and two outputsV1out and V2out. The input I1 is supplied to the base terminal of the transistor T1, the input I2 is applied to the base terminal of the transistor T2. The emitter outputs of transistor T1 and transistor T2 are connected to a common emitter resistor. Thus, the two input signals I1 and I2 will affect the outputs V1out and V2out. The circuit consists of two supply voltages Vcc and Vee, but there is no ground terminal. Even with a single supply voltage, the circuit can operate normally (similarly when using two supply voltages). Consequently, opposite points of positive voltage and negative supply voltage are connected to ground.
Schematic diagram of the work of DT
The operation of the differential amplifier is shown in the diagram in the figure below.
If the input signal (I1) is fed to the basetransistor T1, a positive voltage drop appears across the resistor connected to the transistor transistor T1, which will be smaller. If the input signal (I1) is not applied to the base of the transistor T1, a positive voltage drop appears across the resistor connected to the transistor transistor T1, which will be large.
It can be said that the inverting output coming outthrough the collector terminal of the transistor T1, is based on the input signal I1 supplied to the base terminal T1. If T1 is on, applying a positive value of I1, the current passing through the emitter resistance increases when the emitter current and collector current are almost equal. If the voltage drop across the emitter resistance increases, then the emitter of both transistors goes in the positive direction. If the emitter of the transistor T2 is positive, then the base T2 will be negative, and in this state the current will be less. And there will be less voltage drop across the resistor connected to the collector terminal of transistor T2.
Therefore, for this positiveThe collector of the input signal T2 will go in the positive direction. It can be said that the non-inverting output occurring at the collector terminal of the transistor T2 is based on the input signal fed to the base T1. The differential amplifier receives the output signal between the collector terminals of transistors T1 and T2. From the above circuit diagram it is assumed that all the characteristics of the transistors T1 and T2 are identical, and if the base voltages Vb1 are Vb2 (the base voltage of the transistor T1 is equal to the base voltage of the transistor T2), then the emitter currents of both transistors will be equal (Iem1 = Iem2).
Thus, the total emitter current will be equal tothe sum of the emitter currents T1 (Iem1) and T2 (Iem2). Calculation of the differential amplifier. Iem1 = Iem2 Ie = Iem1 + Iem2 Vev = Vb-Vb em I em = (Vb-Vb em) / Rem. Thus, the emitter current remains unchanged regardless of the value of hfe of transistors T1 and T2. If the resistors connected to the collector terminals T1 and T2 are equal, then their collector voltages are also equal.
Brief description of operation of the operational amplifier
This amplifier (Op-amp, English version) canbe perfect with infinite gain and bandwidth when used in open-loop mode with a typical DC gain greater than 100,000 or 100 dB. Differential amplifier current OU has two inputs, one of which is inverted. The amplified difference of these inputs is output as voltage. The ideal op amp has an infinitely high gain. This should express the symbol of infinity with the new symbol. The op amp operates either with double positive (+ V), or with corresponding negative (-V) power, or it can operate on a single constant voltage supply.
The two main laws related to the OS
They are that such an amplifier hasinfinite input impedance (Z = ∞), which leads to the absence of current flowing into one of its two inputs and zero input bias voltage V1 = V2. The op amp also has zero output impedance (Z = 0). Optical amplifiers determine the difference between the voltage signals applied to their two input terminals, and then multiply them by some predetermined gain (A). This gain (A) is often called - Open Loop Ratio. OA can be connected in two basic configurations - inverting and non-inverting.
For negative feedback if voltagefeedback is in antiphase at the input, the overall gain is reduced. For positive feedback, when the feedback voltage is in the "phase", the input signal of the amplifier increases. By connecting the output back to the negative input connector, 100% feedback is achieved, with the result that the voltage follower circuit (buffer) is obtained with a constant gain of 1 (Unity). By replacing the fixed feedback resistor (Rƒ) for the potentiometer, the circuit will have an adjustable gain factor.
Specifications
Major:
- Zero-sequence input current (inputbias current) in the rest position, different currents can flow at two inputs. This means in practice that the voltage is distorted in the case of signal sources with high internal resistance, since the sources are subjected to different voltage levels.
- Input impedance can be measured againstland at the inputs, provided that the other input is grounded. The disadvantage here is sources with high internal resistance, which are partially loaded with input resistance.
- Input capacitors - parallel capacitorsinput resistors. They have a disturbing effect, especially at high frequencies, since capacitances create additional parallel input resistances, which are frequency dependent. In a differential amplifier, the principle of operation depends on this indicator.
- Low gain (increase signal gain)indicates the gain, which is obtained without feedback. It is determined with a load resistance of 2 kΩ and an output voltage fluctuation of ± 10 V. In practice, this value of 200,000 has never been achieved and is usually 10 times lower.
- The coefficient of deviation of the supply voltage. When the voltage of one volt changes, the offset changes by 0.3 µV. However, with a gain of 300 times, the error increases by 0.1 mV.
- Swing output voltage.OU can never generate the full input voltage at its output. In any case, the maximum output voltage at an input voltage of ± 15 V will be significantly higher than ± 10 V. At normal loads, about ± 13 V and ideal - just 1 V lower than the supply voltage.
- The output impedance is the effective impedance of the alternating current at the output, only for output signals with low and offset output. Practically applicable only in borderline cases.
- Output short circuit current.
- Supply current using an unloaded op amp, 1.7 mA type.
- Performance - power loss, of course,the unloaded op amp is caused by the supply current and is dependent on the operating voltage. The differential amplifier on transistors needs a certain reaction time and degrades the input signal with a jump. This refers to a load of 2 kΩ || 100 pF and strengthening the "unity" (single gain).
- Rate of rise to preventuncontrolled swing. If the output voltage changes to 10 V, the op amp usually takes 5 µs. It becomes critical at high frequencies, as its output is greatly attenuated.
Boundary conditions of use
Major:
- Power supply maximum ± 18V. Most circuits operate at ± 15 V, therefore on the safe side.
- Maximum power loss (dissipatedpower) depends on the version of the case and the maximum allowable temperature. A simple 8-pin plastic case can handle 310 mW, a 14-pin double-row case can work about twice as much.
- Input voltages and differences may be inrange -15 ... + 15 V. Solder. During soldering (soldering), let the terminals heat up to 300 ° C for one minute. Soldering to the terminals is performed not simultaneously, but one after the other, and only after complete cooling of the entire component.
- Short circuit on the output side. According to the manufacturer, the output short circuit can last indefinitely if all the boundary conditions are met.
- Restriction: the case temperature must not exceed 125 ° C, therefore the ambient temperature must not exceed 75 ° C
Differential amplifier using bjt
The principle of its operation is shown in the diagram below.
It is built using two matchingtransistors in the general configuration of an emitter whose emitters are connected to each other. A simple circuit that can amplify small signals between two inputs, but suppress noise signals common to both inputs.
Дифференциальный усилитель на биполярных transistors (BJT) has a unique topology: two inputs and two outputs. Although it is possible to use a signal from only one output, the difference between both outputs provides twice the gain! And this improves common mode suppression (CMR) when the common mode signal is a noise source or a DC offset from the previous step.
Transistor Computer Configuration
Based on input and output data entry methods, differential amplifiers can have four different configurations, as shown below.
- Single phase unbalanced output.
- Single balanced input output.
- Dual input unbalanced output.
- Dual input balanced output.
Circuit diagram of the DC amplifier
When developing analog building blocks(various types of preamplifiers, filters, etc.) it is important, along with the development of modern solutions for deep submicron technologies, to pay attention to new structural solutions of traditional amplifying devices.
Dc differential amplifier(DUPT), its output voltage is proportional to the difference between the two input voltages. This can be represented in the form of an equation as follows: V out = A * ((Vin +) - (Vin-)), where A = gain.
Practical use
In practical schemes, the remote control is used to amplify: pulses through long wires, sound, radio frequencies, control of motors and servomotors, electrocardiograms, information on magnetic drives.
disadvantages
The differential amplifier has a number of disadvantages, somewhat limiting its use in electronics:
- Low input resistance, depending on the resistor, for example, with a weak signal from a thermocouple - the remote control will give an incorrect measurement result.
- Difficult gain, whichwill require changes in the value of the two resistors, which is practically difficult to implement, and the introduction of additional elements (potentiometers or multiplexers) into the circuit will unnecessarily complicate the circuit.
