Fully Differential Op-Amp

Model operational amplifier with fully differential output, that is, not referenced to ground

Library

Integrated Circuits

Description

The Fully Differential Op-Amp block models a fully differential operational amplifier. Differential signal transmission is better than single-ended transmission due to reduced susceptibility to external noise sources. Applications include data acquisition where inputs are differential, for example, sigma-delta converters.

The following diagram shows the internal representation of the amplifier.

Parameters for the circuit components are derived from the block parameters that you provide. The gain of the two voltage-controlled voltage sources (VCVS1 and VCVS2) is set to half of the differential gain value. Similarly the slew rate of each of the voltage sources is set to half of the differential maximum slew rate value. The voltages of the two output ports Vout+ and Vout- are both limited to be within the minimum and maximum output voltages that you specify.

The output voltage for zero differential input voltage is controlled by the common-mode port, cm. If no current is drawn from the cm port by the external circuit, then the output voltage is set to be the average of the positive and negative supply voltages by the resistor ladder of R3a and R3b. Note that the negative supply voltage can be zero, which corresponds to operation when a split supply is not available. The values for the minimum and maximum output voltages that you provide must be consistent with the values for the supply voltages that you provide. So, for example, the maximum output high voltage will be less than the positive supply voltage, the difference corresponding to the number of p-n junction voltage drops in the circuit.

    Note   Physical Network block diagrams do not allow unconnected Conserving ports. If you want to leave pin cm open-circuit, connect it to an Open Circuit block from the Simscape™ Foundation library.

Basic Assumptions and Limitations

This block provides a behavioral model of a fully differential operational amplifier. It does not represent nonlinear effects, such as variation in gain with output voltage amplitude, and the nonlinear nature of the output voltage-current relationship for large load currents.

Dialog Box and Parameters

Gain Tab

Differential gain

The gain applied to a voltage difference between the + and – inputs. The default value is 1000.

Bandwidth

The frequency at which the differential voltage gain drops by 3 dB from its dc value. The default value is 1.5 GHz.

Input Impedance Tab

Differential input resistance

The input resistance seen by a voltage source applied across the + and – inputs. The default value is 1.3 MOhm.

Differential input capacitance

The input capacitance seen by a current source applied across the + and – inputs. The default value is 1.8 pF.

Common-mode input resistance

The input resistance seen by a voltage source applied between ground and the + input, or between ground and the – input. The default value is 1 MOhm.

Common-mode input capacitance

The input capacitance seen by a current source applied between ground and the + input, or between ground and the – input. The default value is 2.3 pF.

Output Limits Tab

Output resistance

The output resistance of either of the outputs with respect to the common-mode voltage reference. Differential output resistance is therefore twice the value of the output resistance R_out. The default value is 1 Ohm.

Minimum output voltage low

The minimum output voltage for either of the two output pins with respect to ground. The default value is -1.4 V.

Maximum output voltage low

The maximum output voltage for either of the two output pins with respect to ground. The default value is 1.4 V.

Differential maximum slew rate

The maximum slew rate of the differential output voltage. The default value is 5000 V/μs.

Output Bias Tab

Common-mode port input resistance

The input resistance seen by a voltage source applied between ground and the common mode port. The default value is 23 kOhm.

Negative supply voltage

The value of the negative supply voltage connected to common-mode bias resistor R3b (see diagram). The default value is -5 V.

Positive supply voltage

The value of the positive supply voltage connected to common-mode bias resistor R3a (see diagram). The default value is 5 V.

Initial Conditions Tab

Initial differential output voltage

The initial differential voltage across the two outputs if the output current is zero. The default value is 0 V.

Ports

The block has the following ports:

+

Noninverting input.

-

Inverting input.

cm

Common-mode port. If you want to leave this pin open-circuit, connect a voltage sensor between the cm port and a reference port.

Vout+

Noninverting output.

Vout-

Inverting output.

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