In this article, we will discuss Differential Amplifier (OPAMP). So far, we have used only one of the operational amplifier inputs to connect to the amplifier, using the input terminal that either "converts" or "does not convert" to amplify a single input signal while the other input is connected to the ground.
However, since a standard transactional amplifier has two inputs, inverting and inverting, we can simultaneously connect signals to both of these inputs and produce another common operational amplifier circuit called the Differential Amplifier.
Basically, as we saw in the first lesson on transactional amplifiers, all op-amps are "Differential Amplifiers" due to +input configurations. However, the output voltage obtained by connecting a voltage signal to one input terminal and another voltage signal to the other input terminal will be proportional to the "Difference" between the two input voltage signals of V1 and V2.
The differential amplifiers then increase the difference between the two voltages, making this type of operational amplifier circuit a Extractor, unlike a aggregator that adds or collects input voltages. This type of transactional amplifier circuit is commonly known as the Differential Amplifier configuration and is shown below:
By connecting each input to 0v of soil in turn, we can use superposition to solve the output voltage Vout. Then the transfer function for a Differential Amplifier circuit is given as follows:
When R1 = R2 and R3 = R4, the above transfer function for the differential amplifier can be simplified to the following statement:
Difference Recipient Amplifier Equation
If all resistors have the same omic value, that is: R1 = R2 = R3 = R4 then the circuit will be the Union Gain Differential Riser, and the voltage gain of the riser will be exactly one or one. Then the output expression will simply be Vout = V2 – V1. Also note that if the V1 input is higher than the V2 input, the output voltage total will be negative, and if it is higher than V2 V1, the output voltage total will be positive.
Differential Amplifier circuit is a very useful op-amp circuit. The circuit obtained by adding more resistance in parallel with the R1 and R3 input resistors can be made as "Add" or "Subtract" the voltages applied to the relevant inputs. One of the most common ways to do this is to connect a "Resistant Bridge", often called the Wheatstone Bridge, to the entrance of the amplifier, as shown below.
Wheatstone Bridge Differential Amplifier
The Standard Differential Amplifier circuit now becomes a differential voltage comparator by "comparing" one input voltage to the other. For example, by connecting one input to a fixed voltage reference installed on one foot of the resistant bridge network and the other to a "Thermistor" or a "Light Dependent Resistance", the amplifier circuit can be used to detect low or high.
Light Dependent Difference Receiver Amplifier
Here, the circuit above acts as a light-activated switch that makes the output relay "ON" or "OFF" when the light level detected by LDR resistance exceeds or falls below a preset value. A fixed voltage reference is applied to the non-inverting input terminal of the op-amp via the R1 – R2 voltage divider network.
The voltage value in V1 adjusts the opening point of op-amps with a feedback ponsiometer used to adjust switching hysteria. This is the difference between the light level for "ON" and the light level for "OFF".
The second leg of the differential amplifier consists of resistance attached to a standard light, also known as LDR, a photodirective sensor that replaces the resistance value (hence its name) with the amount of light in its cell, since the resistance value is a function of lighting. .
LDR can be any standard cadmium-sulfur (cdS) photoiletken cell type, such as common NORP12, which has a resistance range of approximately 500Ω in sunlight and about 20kΩ or more in the dark.
The NORP12 photoconductor cell has a spectral response similar to that of the human eye, making it ideal for use in lighting control type applications. Its photocell resistance is proportional to the light level and decreases with increased light intensity, so the voltage level in the V2 will also change above or below the switching point, which can be determined by the position of the VR1.
Then, by adjusting switching hysteresis using the VR1 ponsiometer to turn on the light level or adjust the setting position and the ponciometer, VR2 can be made a sensitive light-sensitive switch. Depending on the application, the output from the op-amp can directly change the load or use a transistor switch to control a relay or lamps.
It is also possible to detect the temperature using this type of simple circuit configuration by replacing the resistance due to light with a thermist. By changing the positions of vr1 and LDR, the circuit can be used to detect light or darkness or heat or cold using a thermistor.
A significant limitation of this type of amplifier design is that input impedances are lower than other operational amplifier configurations, such as an inverted (single-ended input) amplifier.
Each input voltage source must drive the current only through an input resistance that has less total impedance than the op-amp input. This may be good for a low impedance resource, such as the bridge circuit above, but not very good for a high impedance resource.
One way to overcome this problem is to add a Unity Gain Buffer Amplifier to each input resistance, such as the voltage tracker seen in the previous tutorial. This then gives us a differential amplifier circuit with very high input impedance and low output impedance, since this circuit consists of two non-inverted buffers and a differential amplifier. This then forms the basis for most "Instrumentation Amplifiers".
Instrumentation Amplifiers are very high-gain differential amplifiers with high input impedance and a single-ended output. Instrumentation amplifiers are mainly used in engine control systems to amplify very small differential signals from strain gauges, thermocoupls or current detection devices.
Unlike standard transactional amplifiers, where closed loop gains are determined by a positive or negative external resistant feedback connected between the output terminals and an input terminal, "instrumentation amplifiers" have an internal feedback resistance effectively isolated from the input terminals. since the input signal is applied to two differential inputs, V1 and V2.
The instrumentation amplifier also has a very good common mode rejection rate, cmrr (zero output when V1 = V2) is well above 100dB in DC. Here is a typical example of three op-amp instrumentation amplifiers with high input impedance (Zin):
High Input Impedance Instrumentation Amplifier
Two nonverting amplifiers create a differential entry floor that acts as buffer amplifiers with a gain of 1 + 2R2/R1 for differential input signals and union gain for common mode input signals. Since the A1 and A2 amplifiers are closed loop negative feedback amplifiers, we can expect the voltage in the Va to be equal to the V1 input voltage. Likewise, the voltage in Vb will be equal to the value in V2.
Since op-amps do not receive current in input terminals (virtual soil), three resistances connected to the same current op-amp outputs must flow through the R2, R1 and R2 networks. This means that the voltage at the upper end of the R1 will be equal to V1 and the voltage at the lower end of R1 will be equal to V2.
This resistance produces a voltage drop along R1 equal to the voltage difference between the V1 and V2 inputs, the differential input voltage. Because the voltage in the aggregation connection of each amplifier is equal to the voltage applied to the positive inputs of Va and Vb. .
However, if a common mode voltage is applied to the amplifier inputs, the voltages on both sides of the R1 will be equal and no current will flow from this resistance. Since no current passes through R1 (and therefore does not pass through both R2 resistances), the A1 and A2 amplifiers will operate as union gain trackers (buffers). Since the input voltage at the outputs of the A1 and A2 amplifiers appears differently throughout the three resistance networks, the differential gain of the circuit can only be changed by changing the value of the R1.
The voltage output from the differential op-amp A3, which acts as a extractor, is the difference between the two inputs (V2 – V1). Next, we have a general expression for the overall voltage gain of the instrumentation amplifier circuit:
Instrumentation Amplifier Equation
In the next tutorial on Operational Amplifiers, we will examine the effect of output voltage, Vout when the feedback resistance is replaced by a frequency-dependent reactance in the form of a capacitance. The addition of this feedback capacitance produces a nonlinear operational amplifier circuit called the Integrated Amplifier.