Fully Differential Op-Amp
Operational amplifier with fully differential output, that is, not referenced to ground
Simscape / Electrical / Integrated Circuits
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- 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
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.
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.
+ — Non-inverting input
Electrical conserving port associated with the op-amp non-inverting input.
- — Inverting input
Electrical conserving port associated with the op-amp inverting input.
cm — Common-mode port
Electrical conserving port associated with the op-amp common-mode
port. If you want to leave this pin open-circuit, connect a voltage
sensor between the
cm port and a reference
Vout+ — Non-inverting output
Electrical conserving port associated with the op-amp non-inverting output.
Vout- — Inverting output
Electrical conserving port associated with the op-amp inverting output.
Differential gain — Differential gain
The gain applied to a voltage difference between the + and – inputs.
Bandwidth — Bandwidth
The frequency at which the differential voltage gain drops by 3 dB from its dc value.
Differential input resistance — Differential input resistance
The input resistance seen by a voltage source applied across the + and
– inputs. The default value is
Differential input capacitance — Differential input capacitance
The input capacitance seen by a current source applied across the + and – inputs.
Common-mode input resistance — Common-mode input resistance
The input resistance seen by a voltage source applied between ground and the + input, or between ground and the – input.
Common-mode input capacitance — Common-mode input capacitance
The input capacitance seen by a current source applied between ground and the + input, or between ground and the – input.
Output resistance — 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.
Minimum output voltage low — Minimum output voltage low
The minimum output voltage for either of the two output pins with respect to ground.
Maximum output voltage low — Maximum output voltage low
The maximum output voltage for either of the two output pins with respect to ground.
Differential maximum slew rate — Differential maximum slew rate
The maximum slew rate of the differential output voltage.
Common-mode port input resistance — Common-mode port input resistance
The input resistance seen by a voltage source applied between ground and the common mode port.
Negative supply voltage — Negative supply voltage
The value of the negative supply voltage connected to common-mode bias resistor R3b (see diagram).
Positive supply voltage — Positive supply voltage
The value of the positive supply voltage connected to common-mode bias resistor R3a (see diagram).
Initial differential output voltage — Initial differential output voltage
The initial differential voltage across the two outputs if the output current is zero.