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Intermediate Op Amp ModelCIRCUIT
OPMODEL2.CR Download the SPICE file Just like the basic model, the intermediate opamp model captures your basic opamp stages: Differential Amplifier, Gain / Frequency Response and Output Buffer. However, this SPICE model is beginning to take the shape of those created by op amp manufacturers. Although each manufacturer has its own approach of modeling a particular behavior, most are based on this basic structure. And once you understand this model, you can easily customize or expand it to simulate other advanced op amp characteristics. There's also an Excel spreadsheet available below that calculates component values of the op amp circuit.
op amp BEHAVIORS Here's a roundup of the behaviors modeled.
This model, like many op amp models, has these key features.
OVERALL VOLTAGE GAIN What's the overall gain of the circuit? From differential input to output buffer, you can write the gain as
where
Because a typical model sets KDIFF =1 and KBUFF=1, the open loop gain is simply
DIFFERENTIAL AMPLIFIER What's different about this differential amplifier compared to the basic op amp model? This one uses an actual transistor differential pair in the first stage. Why? As you'll see later, it behaves much like the mechanism that causes slew-rate limiting inside an op amp. But first, let's look at the voltage gain from differential input to differential output.
where gm = IE1 / VT and VT = 25.85 mV at 27 ° C. Bias current I1 spits evenly into each emitter, IE1 = IE2 = I1 / 2, when the base voltages are equal. Typically, RC1 and RE1 are chosen for unity gain. This simplifies calculations. How? Conveniently, the open-loop gain, first pole and slew-rate are all now set by next stage's components - G1, RP1 and CP1.
FREQUENCY RESPONSE The first-pole frequency fp1 provides the classic low-pass filter response of the op amp by itself. Modeled by a simple RC filter, CP1 and RP1, the frequency fp1 is given by
A direct result of fp1 and the open-loop DC gain AOL_DC is the Unity-Gain Frequency, fu – the frequency where AOL runs out of gas and falls to 1 V/V. It can be approximated by fu = AOL_DC ∙ fp1 The Unity-Gain Frequency fu typically has another name, the Gain-Bandwidth Product. The higher the fu, the faster your op amp. Unfortunately, op amp manufacturers don't specify fp1. Fortunately, they do specify fu and Aol so you can easily calculate SLEW-RATE LIMITING What is slew-rate and why is it limited? Slew rate is simply how fast a voltage can change. And here's what limits it. In the voltage gain stage, G1's current charges CP1. This current in G1 is really the difference in the transistor collector currents. For large differential inputs, one transistor turns OFF completely while the other transistor turns ON completely. The ON transistor now carries the maximum current available, I1. This maximum current gets transferred to G1, limiting how fast CP1 gets charged. To a good approximation, this reflects the mechanism inside an op amp! A current source charging a capacitor develops a voltage over time. The rate of voltage change looks like this
For our op amp model, the rate is limited by the maximum transistor current available.
The remaining challenge is setting up G1's output to reflect the difference in collector currents! G1 generates a current controlled by the difference in collector voltages. IG1 = KG1 ∙ ( V(5) - V(6) ) If RC1 = RC2, we can describe the voltage difference using collector currents. V(5) - V(6) = RC1 ∙ ( IC1 - IC2 ) Sticking this into the previous equation gets us IG1 = KG1 ∙ RC1 ∙ ( IC1 - IC2 ) Finally, by choosing
G1's current becomes IG1 = IC1 - IC2
OUTPUT BUFFER The model for the output stage is a simple Voltage-Controlled Voltage Source (VCVS), whose output is described by VEBUF = KEBUF ∙ ( V(10)-V(100) ) where KEBUF = 1. EBUF drives the output through series RO representing the op amp's output resistance. Although not very exciting, at this level of modeling anyway, far more interesting output models await in the Level 3 op amp model where advanced behaviors - current limiting, supply-rail current - will be created.
FLOATING INTERNAL REFERENCE You may have noticed two of the stages, the Voltage Gain and the Output Stage, ride on top of a voltage source EREF at node 100. This voltage, named VREF, provides the internal reference for all stages in this model. It automatically "floats" the node midway between the supply rails. Resistors RREF1 and REF2, strung across the supply rails, develop this midpoint voltage at node 103.
Early op amp models did not have this midpoint voltage; all internal stages were referenced to 0V (ground). Although adequate for symmetric dual-supply applications, simulations may have been inaccurate for single, asymmetric, or unusual power supply configurations.
OP AMP SPECS TO COMPONENT VALUES How do you go from op amp specs to SPICE model? Just a few simple calculations and your model is ready to go.
OPEN-LOOP RESPONSE With no feedback components, let's try out the op amp model, OPMODEL2.CIR. For convenience, the model is defined as a subcircuit, OPAMP2. VS drives the positive input; R1 = 1 Ω grounds the negative input. Although not shown, this model needs its supply rails powered by VCC and VEE.
Run an AC Analysis and plot the output V(3). How does the SPICE model perform? Does the level at low frequencies meet the spec of AOL_DC = 100k V/V? Does the output fall to 0.707 of maximum at fp1 = 10 Hz? To get a better view, change the Y-Axis Settings to a log scale. And finally, does V(3) drop to unity at fu = 1MHz?
CLOSED-LOOP RESPONSE Time to wrap our op amp model inside some feedback and take it out for a spin.
For a non-inverting amplifier with unity gain, remove R1 by adding a "*" at the beginning of its statement and add R2 = 1 Ω by removing the "*" at the beginning of its statement. VS generates a 1 V step input voltage. Run an Transient Analysis and plot the input V(1) and output V(3). How fast does the output reach 1 V? CIRCUIT INSIGHT Does the slew-rate of this model max out at the expected 0.6 V/μs? The test is simple: did it take 1 μs for the output to reach 0.6 V? For larger input voltages like 5V, the slew-rate limit should remain the same. And so, regardless of the larger input, the output keeps chugging toward its final level at the same rate. During the slew-rate limited interval, check out peak current in G1 by plotting I(G1); it should be equal to I1 = 1 mA. This is the max current available to charge CP1. For smaller input voltages - where the output does not exceed the slew-rate limit - the output settles quicker. Change VS to something smaller like 100MV. Did the circuit respond faster? HANDS-ON DESIGN Add some gain by including R1 in the circuit. Or, change the circuit into an inverting amplifier. Vary the input voltage and gain. What happens to the output waveform and settling time?
OP AMP MODEL - LEVEL 3 If you made it through this op amp modeling adventure - congratulations! - a round of cool ones for everyone! Level 2 rounded up several key op amp behaviors, but left plenty of others behind. Next, we'll build up this model with advanced behaviors - one at a time - with the Level 3 op amp model.
Looking at the vast array of op amp models out there, it seems each manufacturer or SPICE modeler has their own idea of the "best" approach. We'll try to cover some of the more popular ones and capture, at least the spirit, of other approaches. Hopefully, it will give you the tools to understand existing models and create your own if needed.
SIMULATION NOTES For a description of all op amp models, see
Op Amp Models.
SPICE FILE Download the file or copy this netlist into a text file with the *.cir extension. OPAMP2.CIR - OPAMP MODEL (LEVEL 2) * * SIGNAL SOURCE VS 1 0 AC 1 PWL(0US 0V 0.01US 1V 100US 1V) * * POWER SUPPLIES VCC 10 0 DC +15V VEE 11 0 DC -15V * R1 0 2 1 *R2 2 3 1 XOP 1 2 3 10 11 OPAMP2 RL 3 0 100K * * * OPAMP MACRO MODEL (INTREMEDIATE LEVEL) * * IN+ IN- OUT VCC VEE .SUBCKT OPAMP2 1 2 81 101 102 Q1 5 1 7 NPN Q2 6 2 8 NPN RC1 101 5 95.49 RC2 101 6 95.49 RE1 7 4 43.79 RE2 8 4 43.79 I1 4 102 0.001 * * OPEN-LOOP GAIN, FIRST POLE AND SLEW RATE G1 100 10 6 5 0.0104719 RP1 10 100 9.549MEG CP1 10 100 0.0016667UF * *OUTPUT STAGE EOUT 80 100 10 100 1 RO 80 81 100 * * INTERNAL REFERENCE RREF1 101 103 100K RREF2 103 102 100K EREF 100 0 103 0 1 R100 100 0 1MEG * .model NPN NPN(BF=50000) * .ENDS * * ANALYSIS .TRAN 0.1US 5US .AC DEC 5 0.1 10MEG * * VIEW RESULTS .PRINT TRAN V(3) .PRINT AC V(3) .PROBE .END
REFERENCES
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