Operational Amplifiers (OpAmps)

Operational Amplifiers (OpAmps)

• OpAmps are transistor amplifiers with two inputs and one output
• They are manufactured as integrated circuits (ICs)
• They have small dimensions and are versatile.

Applications

• Used in control engineering as setpoint comparators and controllers
• Used in measurement technology as measuring amplifiers with high input impedance
• Used in digital technology for pulse circuits, digital-to-analog converters, threshold switches, and as low-frequency preamplifiers, constant current and voltage generators.

OpAmp Structure

• OpAmps comprise multiple amplifier stages that are galvanically coupled
• The input stage is always a differential amplifier with two inputs
• The driver stage is a voltage amplifier
• The output stage is typically a push-pull amplifier.

Input Amplifier Stage

• Differential amplifier stages use positive and negative operating voltages
• Transistors receive emitter currents from a common constant current source
• Transistors' operating points are set so that with Uᵢd = 0V, the emitter and collector currents are equal, resulting in Uₐ = 0V
• A positive control voltage Uᵢ₁ at E1 of K1 causes K1 to conduct more.
• Results in less current through K2
• I1 increases and I2 decreases proportionally, while their sum remains constant
• The collector of K2 becomes more positive
• A positive input voltage Uᵢ₁ leads to a negative output voltage Uₐ
• Activation of K2 via E2 with a positive input signal Uᵢ₂ causes the potential at the collector of K2 to assume a negative value
• Creates a positive voltage Uₐ at the output of the differential amplifier
• A driver stage and usually a push-pull output stage follow the differential amplifier.

OpAmp Connections

• E1 is called the inverting input because its activation causes a change of sign of the output voltage Uₐ.
• Indicated with a minus sign in circuit diagrams
• E2 is the non-inverting input and is marked with a plus sign
• The output voltage Uₐ is measured between the output and the common connection (0V) of both operating voltages, which can range between ±2V and ±18V, depending on the OP
• Schematics typically only show the two inputs and the output.

OpAmp Characteristics

• OpAmps have nearly ideal amplification properties
• Amplify both DC and AC voltage
• A positive differential input voltage Uᵢd (Uᵢ₁ > Uᵢ₂) results in a negative output voltage Uₐ, and vice versa
• The amplification behavior is represented by its transfer characteristic
• The OpAmp saturates at differential input voltages below -0.15 mV and above +0.15 mV
• Amplification is linear between these values
• Outside the control range, the OpAmp is overdriven and supplies approximately +Ub or -Ub at the output.

Null Adjustment (Offset Compensation)

• Without compensation, the output voltage Uₐ is usually not exactly zero, even when the differential input voltage Uᵢd = 0
• The output voltage arises from asymmetries, such as unequal current amplification factors of the transistors.
• A trimmer potentiometer R1 can be used to adjust the output of the LM 741 to Uₐ = 0V via the input offset voltage.
• The BI-MOS operational amplifier CA 3140 has an input offset voltage of max. 5 mV, compensable with a 100 kO trimmer potentiometer
• The CA 3140 has a MOS-FET input stage and a bipolar output stage
• It has the same pin assignment as the older LM 741 and can directly replace it.

Frequency Compensation

• As frequency increases, the open-loop voltage amplification V₀ decreases
• The output voltage experiences a phase shift, causing the OP to tend toward high-frequency oscillations
• Newer OPs like the CA 3140 have internal frequency compensation
• Older OPs like the LM 709 require external frequency compensation.

OpAmps with External Circuitry

• OpAmps can be used as amplifiers when the high open-loop voltage amplification factor is reduced to required amplification values
• External circuitry with components, usually resistors and capacitors, enables an OP to be adapted to almost any desired function and amplification.

Inverting Amplifier

• In the inverting OP, part of the output voltage is fed back in opposite phase to the inverting input via the feedback resistor Rķ
• Feedback occurs when part of the output voltage is fed back to the input through an electrical connection
• A positive input voltage Uᵢ causes the differential input voltage Uᵢd to increase
• Uᵢd is amplified by the open-loop voltage amplification factor V₀
• The negative output voltage Uₐ increases rapidly according to the slew rate decreases Uᵢd via the resistor RK
• Uₐ increases until Uᵢd becomes practically zero and the OP is balanced
• If the signal is out of phase, there is negative feedback
• If the feedback occurs in phase, there is positive feedback

Considerations for OP Circuits

• For calculating OP circuits with negative feedback:
• Differential input voltage Uᵢd ≈ 0 V
• Input quiescent current is negligible, Iᵢ ≈ 0 A

Implications

• Since the differential input voltage Uᵢd ≈ 0V, the input E1 with its current summing point S (virtual ground) is practically at the same ground potential as the non-inverting input E2
• Therefore, Ik + I = 0
• The voltage amplification factor V is derived from the resistance ratio RK/R
• A positive input voltage U leads to a negative output voltage U and vice versa
• The minus sign indicates that it is an inverting OP.

Non-Inverting Amplifiers

• Possess a very large input resistance and a much smaller output resistance
• input Voltage U and output voltage U have the same sign
• Suited as measuring amplifiers
• Has a Voltage divider at the total voltage U and the partial voltage U across the resistors

Analog Circuits with OpAmps

• Summing amplifiers add and amplify multiple input voltages
• There is a phase shift of 180° between same-phase input voltages and the output voltage
• Used in digital-to-analog converters

Impedance Converter

• The output voltage U has the same magnitude
• The input resistance is very high and the output resistance is low
• Match a high internal resistance of a signal source to a small load resistance

Subtraction Amplifier

• Used in measurement for amplifying the differential voltage of a bridge circuit
• Often R₁ = R and R₂ = RK

Digital Circuits with OpAmps

• Amplifying analog signals is a typical application
• Application extends to the field of digital technology

Astable Multivibrators

• Used to generate square-wave voltages
• Consist of a few components: an operational amplifier, three resistors and a capacitor
• Form a comparator with the resistors R₁ and R₂
• If a triangular voltage is applied to the inverting input of the comparator, the voltage UR2 = (U2R2)/(R1 + R2) is applied to R2
• If U₁ exceeds +UR2max, then U₂ and UR2 become smaller, until UR2 reaches the value –UR2max
• If U₁ becomes more negative from t2, then U₂ flips again
• In the astable multivibrator, the comparator switches when the voltage difference at the inverting and non-inverting input is zero
• If the capacitor C₁ is discharged and the voltage U₂ is maximal, then the capacitor C₁ is charged via the resistor R₃
• If the capacitor voltage U₁ reaches the value of UR2, then the circuit flips

Monostable Multivibrators

• Based on the basic circuit of the astable multivibrator
• The astable multivibrator is extended by the diode R6, which limits the positive voltage at the capacitor C₁
• If the output voltage U₂ is positive, the circuit is in a stable state
• At a negative voltage flank U₁ at the circuit input, which puts the non-inverting input of the operational amplifier to a value below 0.7 V, the circuit flips

Schmitt Trigger

• Based on an inverting comparator
• Used in twilight switches
• Switch depending on the input voltage in a specific output state
• The switching voltage U₁ is calculated using a formula