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# Output Voltage Limit

## CIRCUIT

Schematic: Op_Amp_vlim.asc
Op Amp Symbol: Opamp_3.asy
Op Amp Shematic: Opamp_3.asc
Right Click on filename, select "Save link as...",

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## SPEC IT

 Intro The system provides you with +/12V power supply rails. That's good news because you need to deliver a +/-10V signal. However, your test shows the output slams into a limit less than 10V. How can you solve this issue? Definition Vlim - defines how close that the output voltage can swing to within the supply rails (Vcc or Vee). Vo_max+ = Vcc - VlimVo_max- = Vee + Vlim Design Goal Deliver a 10V signal into a 10k load Design Spec Vo ≥ 10.0V

## DESIGN IT

• Circuit Design
• Unity Gain: Kcl = 1V/V
• R2=10k, R1=open
• Vcc = 12V, Vee=-12V
• Op Model Param
• Aol=1e6 fu=1e6 slew=5 vlim=2.5 ilim=0.04 ro=1
• Output can swing within 2.5V of the rails.
• Circuit Test
• Drive the input to just over 10V
.step param Vstep list 1V 5V10.1V
Repeats simulation for 3 levels of Vstep.
• Expected vo ≥ 10V

## TEST IT

• Run a Transient simulation. (.TRAN)
• Plot v(vs) and v(vo) and measure the output's highest voltage level.
• Did it meet spec?
• If vo ≥ 10V, then PASS, else FAIL
•  DESIGN ISSUE: The output voltage fails to meet spec!

## SOLVE IT

• Incrementally decrease Vlim.
• Rerun the SPICE sim.
• What Vlim is needed to meet spec?

## THEORY REFRESH

• The output voltage can swing only so close to the power supply rail VCC. Why?
Let's take a look at a typical positive output structure..

• What components are between the output and VCC?
• R5 - the resistor used to limit max output current.
• Q7 - the output transistor that delivers positive output current.
• Vlim can be decribed by .
• Vlim = (VCC - Vout) = Iout*R1 + VBE_Q7 + V_ISRC
• where
• Iout*R5 - voltage across current limiting resistor
• VBE_Q7 - base-emmiter voltage of Q7
• V_ISRC - min voltage across current source (150uA)
• The specific Vlim for an op amp device will vary depending on its output structure and actual devices.
A similar behavior holds true for the output swing toward the negative rail VEE.
• What happened to our original design?
• Vlim = 2.5V
• The expected max output would be
Vo_max = VCC - Vlim = 12V - 2.5V = 9.5V
• Let's solve it
• What Vlim is required for Vo_max = 10.0V?
• Solve Vo_max = VCC - Vlim  for Vlim
• Then calculate Vlim = VCC - Vo_max
• Retest the circuit with the new Vlim. Did your circuit meet spec?

Select an op amp that meets or exceeds your required Vlim.

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