Department of CNET Electronic Circuit II Chapter 2 Mohammed Abdul Gaffar

Electronic Circuit-II Chapter 2

Topics in the Chapter 2 Operational Amplifier input modes and parameters. Single ended mode OPAMP. Double Ended mode OPAMP. Parameters of OPAMP. Definition of CMRR. Common Mode i/p voltage. I/p offset voltage. Input Bias current. I/p impedance. I/p offset current. Output Impedance. Slew rate.

Operational Amplifier input modes and parameters. Depending on the input signal connection OP-AMP are divided in to two categories. OP-AMP Double Ended Mode Single Ended Mode Common Mode Inverting Mode Non Inverting Mode Differential Mode

Single Ended Mode OP-AMP. If only one signal is connected to one of the input terminals of OP-AMP and other being grounded then it is called Single ended OP-AMP.

Op-Amp Input Modes and Parameters 1 Op-Amp Input Modes and Parameters 1. Input Signal Modes – Signal-Ended Input When an op-amp is operated in the single-ended mode, one input is grounded and signal voltage is applied only to the other input as shown in Figure. In the case where the signal voltage is applied to the inverting input as in part (a), an inverted amplified signal voltage appears at the output. In the case where the signal voltage is applied to the Noninverting input with the inverting input grounded, as in part (b), a Non Inverting amplified signal voltage appears at the output.

Circuit Diagram for Inverting (a) & Noninverting (b) OP-AMP.

Double Ended Mode OP-AMP. If two signals are connected to the two input terminals of OP-AMP then it is called Double ended OP-AMP.

Op-Amp Input Modes and Parameters 2 Op-Amp Input Modes and Parameters 2. Input Signal Modes - Differential Input In the Differential Mode, two opposite-polarity (out-of-phase) signals are applied to the inputs, as shown in Figure. This type of operation is also referred to as double-ended. The amplified difference between the two inputs appears on the output.

Op-Amp Input Modes and Parameters 3 Op-Amp Input Modes and Parameters 3. Input Signal Modes - Common-Mode Input In the Common Mode, two signal voltages of the same phase, frequency, and amplitude are applied to the two inputs, as shown in Figure. When equal input signals are applied to both inputs, they cancel, resulting in a zero output voltage. This action is called common-mode rejection.

Identify the type of input mode for each op-amp in Figure.

Parameters of OP-AMP.

Common-Mode Rejection Ratio The ability of an amplifier to amplify differential signals and reject common-mode signals is called the common-mode rejection ratio (CMRR). CMRR is defined as where Aol is the open-loop differential-gain and Acm is the common-mode gain. CMRR can also be expressed in decibels as

Voltage and Current Parameters VO(p-p): The maximum output voltage swing is determined by the op-amp and the power supply voltages VOS: The input offset voltage is the differential dc voltage required between the inputs to force the output to zero volts IBIAS: The input bias current is the average of the two dc currents required to bias the differential amplifier IOS: The input offset current is the difference between the two dc bias currents

Impedance Parameters ZIN(d) : The differential input impedance is the total resistance between the inputs ZIN(cm) : The common-mode input impedance is the resistance between each input and ground Zout: The output impedance is the resistance viewed from the output of the circuit.

Other Parameters Slew rate: The slew rate is the maximum rate of change of the output voltage in response to a step input voltage Example: Determine the slew rate for the output response to a step input. Solution: = 6 V/ms