OP AMPS

Op-amp

Op-amp

A voltage amplifying device that can perform different operations.

Most useful device in analog circuitry

Op-amp is the most useful device in analog circuitry.

CMRR (Common-Mode Rejection Ratio)

A parameter that measures the performance of the op-amp.

Lower voltages and power

Op-amp can operate at lower voltages and power.

Negative feedback principle

The principle of negative feedback demonstrates the usefulness of the op-amp.

Effects of negative feedback

Negative feedback connected to an op-amp changes its bandwidth, cutoff frequency, and gain.

Operational Amplifiers (op amp)

Very high gain amplifiers with very wide bandwidth, very high input impedance (typically a few megaohms), and low output impedance (less than 100 ohms).

Ideal op-amp

Would have infinite voltage gain, infinite input impedance, and zero output impedance.

Input of an op amp

Typically a differential amplifier, containing a number of amplifier stages to achieve a very high voltage gain.

Op amp inputs

Two inputs: Non-inverting input (with + sign) and inverting input (with – sign).

Amplification of input signals

Only the voltage difference between the two inputs is practically amplified by the op amp, and signals common to both inputs are only slightly amplified.

Op amp outputs

Could have one output or two outputs.

Power supply for op amps

Typically use a positive and a negative power supply.

Differential Amplifier (Input Stage)

Provides amplification between the difference of the two input voltages.

Voltage Amplifier (Gain Stage)

Provides additional gain.

Push-Pull Amplifier (Output Stage)

Provides better power output. Operational capabilities of Op-Amp

Current - Current "in" and Current "out"

Describes a current-controlled current source where both the input and output are currents.

Transconductance - Voltage "in" and Current "out"

Describes a voltage-controlled current source where voltage is the input and current is the output.

Transresistance - Current "in" and Voltage "out"

Describes a current-controlled voltage source where current is the input and voltage is the output.

Common Mode Operation

The same input signal/s is applied to both inputs for op amp common mode operation.

Ideal Output for Common Mode

Ideally, the output is 0 volt because the difference of the two inputs is 0 volt.

Amplification of Input Signals

Only the difference between the two input signals is significantly amplified by an op amp.

Reality of Output Voltage

In reality, there is a relatively low output voltage because the circuit components are not exactly matched.

Common Mode Rejection

Common mode rejection is the ability of an op amp to reject the common signals at its inputs.

Noise Reduction

Since noise is typically common to both inputs, the differential connection of op amps typically reduces or attenuates noise significantly. This is one of the most important features of op amps.

Common Mode Rejection Ratio (CMRR)

The ability of an amplifier to reject common mode signals.

Ideal Behavior of Op-Amp

Ideally, an op-amp provides a very high gain for differential-mode signals and zero gain for common-mode signals.

Practical Behavior of Op-Amp

Practical op-amps exhibit a very small gain (less than 1) for common-mode signals, while providing a high open-loop differential voltage gain (several thousands).

Performance Measure of Op-Amp

The higher the open-loop gain with respect to common mode gain, the better the performance of the op-amp in terms of rejection of common-mode signals.

CMRR Calculation for Op-Amp

The CMRR is calculated as the ratio of the open-loop differential voltage gain (A_ol) to the common-mode gain (A_cm).

Open Loop Voltage Gain (AVD)

An op amp specification indicating the voltage gain when there is no feedback resistor.

Comparison of Open Loop and Closed Loop Gain

The open loop gain is very high compared to the closed loop gain, which is the gain of the op amp circuit when there is a feedback resistor.

Benefits of Feedback Resistor

The reduction in voltage gain due to the feedback resistor results in the following benefits: Amplifier voltage gain is more stable. Increased input impedance. Reduced output impedance. Increased or better frequency response.

Maximum Output Voltage Swing (VO(p-p))

Refers to the peak-to-peak output voltage swing of an op-amp.

Quiescent Output Voltage

When there is no input voltage, ideally, the output of an op-amp is zero (quiescent output voltage).

Ideal Limits of Output Voltage Swing

The ideal limits of the peak-to-peak output signal are the ± DC Supply (±Vs), but in reality, the op-amp never reaches the maximum peak-to-peak output voltage.

Variation of Output Voltage with Load Resistance

The output voltage of an op-amp also varies with the load resistance connected to its output terminal.

Single DC Voltage Supply Applications

In some applications, op-amps do not use both the positive and negative DC supply. One example is the analog-to-digital converter that only uses a single DC voltage source.

Output Range for Single DC Voltage Supply

In this case, the op-amp output is designed to operate between ground and a full scale output that is near the positive supply voltage.

Input Offset Voltage

Refers to the small DC output voltage (VOUT(error)) of an op-amp when there is no differential input voltage, despite the input being ideally zero volts. The differential DC voltage required between the inputs to force the output to zero volts.

Primary Cause of Input Offset Voltage

The primary cause of input offset voltage is a slight mismatch of the base-emitter voltages of the differential amplifier input stage of an op-amp.

Ideal Value of VOS

Ideally, the input offset voltage (VOS) should be zero.

Practical Range of VOS

In practical op-amps, the input offset voltage (VOS) is typically in the range of 2 mV or less.

Input Offset Voltage Drift

The offset voltage drift is defined as how much current change occurs in the input offset voltage for each degree change in temperature.

Typical Value of Voltage Drift

The typical value of the voltage drift is in the range of 5 μV to 50 μV per degree Celsius.

Relation Between Offset Voltage and Drift

Op-amps with a higher nominal value of input offset voltage exhibit a higher drift.

Input Bias Current

The DC current required by the input of the amplifier to properly operate the first stage of an op-amp.

Input Impedance (Zin)

The resistance presented by the inputs of an op-amp.

Differential Input Impedance (Zin(d))

Total resistance between inverting and non-inverting input. It can be measured by determining the change in bias current for a given change in differential input voltage.

Common-Mode Input Impedance (Zin(cm))

Total resistance between each input terminal and ground. It can be measured by determining the change in bias current for a given change in common-mode input voltage.

Input Offset Current (IOS)

The absolute value of the difference of the input bias currents in a practical op-amp.

Calculation of IOS

IOS is calculated as the absolute value of the difference between the two input bias currents, defined by the equation: IOS = |Iin1 - Iin2|.

Effect of IOS on Output Voltage

The error created by IOS is amplified by the gain Av of the op-amp and appears in the output as VOUT(error) = Av * IOS * Rin.

Output Impedance (Zout)

The impedance measured across the output terminal of the op-amp.

Slew Rate

The maximum rate of change of the output voltage in response to a step input voltage. It is dependent upon the high-frequency response of the amplifier stages within the op-amp.

Formula for Slew Rate

Slew rate (SR) is calculated as the ratio of the change in output voltage (ΔVout) to the change in time (Δt): SR = ΔVout / Δt.

Frequency Response

The range of frequencies over which an op-amp can effectively amplify signals.

Low Frequency Response

response of op-amps extends down to 0 Hz (DC) due to the absence of coupling capacitors.

Input Offset

Component 1 of the μA-741, representing the input offset.

Inverting Input

Component 2 of the μA-741, representing the inverting input.

Non-Inverting Input

Component 3 of the μA-741, representing the non-inverting input.

-Vcc Supply

Component 4 of the μA-741, representing the negative voltage supply.

Input Offset

Component 5 of the μA-741, representing the input offset.

Output

Component 6 of the μA-741, representing the output.

+Vcc Supply

Component 7 of the μA-741, representing the positive voltage supply.

NC (Not Connected)

Component 8 of the μA-741, indicating a pin that is not connected.

Negative Feedback

A concept widely used in electronics, particularly in op-amp applications, where the output voltage of an amplifier is returned to the input with a phase angle that opposes the input signal.

Internal Inversion

The process within negative feedback where the feedback voltage (Vf) is 180 degrees out of phase with the input voltage (Vin).

Opposing Input Signal through Negative Feedback

A fundamental technique used in electronic circuits, particularly in amplifiers like op-amps, to improve stability, linearity, distortion reduction, and overall performance. It is a powerful tool that allows engineers to design reliable, high-performance electronic systems for a wide range of applications.

Negative Feedback

The open-loop gain of an op-amp is usually very high, resulting in saturated output states for very small input voltages.

Benefits of Negative Feedback

Negative feedback reduces and controls the closed-loop gain (Acl) of the op-amp, allowing it to perform linear functions. It also provides stable voltage gain and control of the input and output impedances, as well as the amplifier bandwidth.

Output Saturation

However, op-amps have limits to the voltages they can output. If the calculated output voltage exceeds this limit, the op-amp saturates.

Controlled Closed-Loop Gain

Negative feedback reduces the effective gain of the op-amp circuit by feeding back a portion of the output voltage to the input with a phase that opposes the input signal.

Stable Voltage Gain

Negative feedback stabilizes the voltage gain of the op-amp circuit, preventing it from fluctuating excessively with changes in input or environmental conditions.

Trade-off between Gain and Bandwidth

In high-gain amplifiers, the gain-bandwidth product limits the available bandwidth. This means that as the gain increases, the available bandwidth decreases, and vice versa.

Bandwidth

In the context of amplifiers, such as operational amplifiers (op-amps), ___ specifies the frequency range over which the amplifier can provide its specified gain or perform its intended function accurately.

Three Basic Configurations

Noninverting Amplifier, Voltage Follower, Inverting Amplifier

Noninverting Amplifier

A configuration where the output signal is in-phase with the input signal.

Noninverting Amplifier Configuration

A configuration where the input signal is applied to the noninverting input and the output is fed back to the inverting input through a feedback circuit formed by resistors 𝑅𝑖 and 𝑅𝑓.

Feedback Circuit

A circuit formed by resistors 𝑅𝑖 and 𝑅𝑓 in a noninverting amplifier configuration. These resistors form a voltage-divider circuit that reduces the output voltage 𝑉𝑜𝑢𝑡 and connects the reduced voltage 𝑉𝑓 to the inverting input.

Voltage Follower

A configuration where the output voltage follows the input voltage.

Inverting Amplifier

A configuration where the output signal is 180 degrees out of phase with the input signal.

Closed-Loop Voltage Gain (Acl)

The voltage gain with an external feedback circuit. The amplifier consists of the op-amp and an external negative feedback circuit connecting the output to the inverting input.

Voltage Follower

A configuration where the output voltage is fed back to the inverting input by a straight connection.

Unity Gain

The voltage follower has a ___, meaning the closed-loop voltage gain (Acl) equals 1.

Values in Voltage Follower

the output voltage (Vo) equals the input voltage (Vi); Acl = 1

Inverting Amplifier Input

The input signal is applied through a series input resistor 𝑅𝑖 to the inverting input.

Feedback in Inverting Amplifier

The output is fed back through resistor 𝑅𝑓 to the inverting input.

Noninverting Input in Inverting Amplifier

The noninverting input is connected to the ground.

Virtual Ground in Inverting Amplifiers

In inverting amplifiers, the operational amplifier's negative feedback strives to make the voltage at the inverting input equal to the voltage at the non-inverting input. In the ideal case, these two input voltages are equal. Basically ang output voltage ay nagiging negative paglagay sa - ng opamp making the difference 0

Frequency Response

Indicates how the voltage gain changes with frequency.

Phase Response

Indicates how the phase shift between the input and output signal changes with frequency.

Bandwidth Limitations

Refers to the limitations in the frequency range over which a system can effectively operate.

Midrange Gain

At frequencies below the critical frequency, the gain remains relatively constant.

Roll-off Behavior

Beyond the critical frequency, the gain starts decreasing, resulting in the roll-off behavior.

3db Open-Loop Bandwidth

In general, the bandwidth is the difference of the high cut-off frequency and low-cut frequency. It is also the frequency range between points where the gain is 3𝑑𝐵 less than the midrange frequency.

Bandwidth in Op-Amps

Since there is no critical low frequency in an op-amp, the bandwidth is the same as the critical high frequency. Therefore, BW = fch.

RC Lag Circuits in Op-Amps

Respnsible for the roll-off in gain as the frequency increases by introducing frequency-dependent behavior. This occurs because the effectiveness of the RC circuits diminishes at higher frequencies due to limited charging or discharging time of the internal capacitances.

RC circuit relation with input and output signal

causes propagation delay resulting to phase shift between input and output signal.

BASIC OP AMP PROPERTIES

Very high open-loop voltage gain, very high input impedance, very low output impedance