I wanted to determine a ballpark number for the power efficiency of op-amps as amplifiers, so I decided to make a simple LTSpice simulation. Having no particular op-amp preference, I chose the first one: LT1001.
Diagram and simulation results attached. (I didn't see a max-current spec on the op-amp datasheet, but I'm assuming it can source 4-8mA.) "Ix(u1:6)" is the output pin. From the LT1001 datasheet, Vsupply+ and Vsupply- are "Ix(u1:7)", "Ix(u1:4)". The current draw at the power supply pins does not match that produced at the output; in fact the current from the supplies is fixed. This probably explains why the simulation indicates that the op-amp sources 16mW (rather than dissipates) when the load is 1k; the op-amp supposedly dissipates 8mW when the load is 3k. (I'm getting these numbers by hovering the mouse over the op-amp symbol, but the numbers are good about summing to zero so they seem somewhat sensible.)
Is this just a failure/limitation of the op-amp modeling, or am I doing something wrong? Assuming it is just the Spice model, could you offer suggestions (e.g. alternate model) for how to make it work?
(with 1k load resistor)
--- Operating Point ---
V(railhigh): 10 voltage
V(raillow): -10 voltage
V(input): -2 voltage
V(n001): 1.22653e-005 voltage
V(output): 6.00005 voltage
I(Rload): 0.00600005 device_current
I(Rfeedback): -0.00200001 device_current
I(Rin): -0.00200001 device_current
I(Vinput): 0.00200001 device_current
I(Vlow): 0.0015999 device_current
I(Vhigh): -0.00159987 device_current
Ix(u1:3): 4.8918e-010 subckt_current
Ix(u1:2): 8.87816e-010 subckt_current
Ix(u1:7): 0.00159987 subckt_current
Ix(u1:4): -0.0015999 subckt_current
Ix(u1:6): -0.00800006 subckt_current
(with 3k load resistor)
--- Operating Point ---
V(railhigh): 10 voltage
V(raillow): -10 voltage
V(input): -2 voltage
V(n001): 1.25712e-005 voltage
V(output): 6.00005 voltage
I(Rload): 0.00200002 device_current
I(Rfeedback): -0.00200001 device_current
I(Rin): -0.00200001 device_current
I(Vinput): 0.00200001 device_current
I(Vlow): 0.0015999 device_current
I(Vhigh): -0.00159987 device_current
Ix(u1:3): 4.89177e-010 subckt_current
Ix(u1:2): 8.8782e-010 subckt_current
Ix(u1:7): 0.00159987 subckt_current
Ix(u1:4): -0.0015999 subckt_current
Ix(u1:6): -0.00400003 subckt_current
Diagram and simulation results attached. (I didn't see a max-current spec on the op-amp datasheet, but I'm assuming it can source 4-8mA.) "Ix(u1:6)" is the output pin. From the LT1001 datasheet, Vsupply+ and Vsupply- are "Ix(u1:7)", "Ix(u1:4)". The current draw at the power supply pins does not match that produced at the output; in fact the current from the supplies is fixed. This probably explains why the simulation indicates that the op-amp sources 16mW (rather than dissipates) when the load is 1k; the op-amp supposedly dissipates 8mW when the load is 3k. (I'm getting these numbers by hovering the mouse over the op-amp symbol, but the numbers are good about summing to zero so they seem somewhat sensible.)
Is this just a failure/limitation of the op-amp modeling, or am I doing something wrong? Assuming it is just the Spice model, could you offer suggestions (e.g. alternate model) for how to make it work?
(with 1k load resistor)
--- Operating Point ---
V(railhigh): 10 voltage
V(raillow): -10 voltage
V(input): -2 voltage
V(n001): 1.22653e-005 voltage
V(output): 6.00005 voltage
I(Rload): 0.00600005 device_current
I(Rfeedback): -0.00200001 device_current
I(Rin): -0.00200001 device_current
I(Vinput): 0.00200001 device_current
I(Vlow): 0.0015999 device_current
I(Vhigh): -0.00159987 device_current
Ix(u1:3): 4.8918e-010 subckt_current
Ix(u1:2): 8.87816e-010 subckt_current
Ix(u1:7): 0.00159987 subckt_current
Ix(u1:4): -0.0015999 subckt_current
Ix(u1:6): -0.00800006 subckt_current
(with 3k load resistor)
--- Operating Point ---
V(railhigh): 10 voltage
V(raillow): -10 voltage
V(input): -2 voltage
V(n001): 1.25712e-005 voltage
V(output): 6.00005 voltage
I(Rload): 0.00200002 device_current
I(Rfeedback): -0.00200001 device_current
I(Rin): -0.00200001 device_current
I(Vinput): 0.00200001 device_current
I(Vlow): 0.0015999 device_current
I(Vhigh): -0.00159987 device_current
Ix(u1:3): 4.89177e-010 subckt_current
Ix(u1:2): 8.8782e-010 subckt_current
Ix(u1:7): 0.00159987 subckt_current
Ix(u1:4): -0.0015999 subckt_current
Ix(u1:6): -0.00400003 subckt_current
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