IC Design Interview 6: Op-amp Circuits

You must know your op-amp circuits to do analog circuit design! During an interview, you even have to derive the operation equations. Each of these derivations starts with the result in bold (because you must just know that) followed by the derivation.

Inverting Op-Amp

V_out = -(R₂/R₁)V_in

V_in/R₁ = -V_out/R₂
-(R₂/R₁)V_in = V_out

Non-Inverting Op-Amp (Buffer)

V_out = V_in(1+R₁/R₂) = V_in((R₂+R₁)/R₂)

V- = V_out R₂/(R₂+R₁)  [Voltage Divider]
Assume V- = V_in
V_in = V_out R₂/(R₂+R₁)
(R₂+R₁)/R₂ V_in = V_out
The special case of R₂ being open (infinite resistance) creates a Unity Gain Buffer

Difference Amplifier

V_out = R₂/R₁ (V_B-V_A)

Assume V- = V+
V+ = V_B R₂/(R₁+R₂)   [Voltage Divider]
Since I₁=I₂                 [Kirchoff's Current Law]
(V_out-V-)/R₂ = (V--V_A)/R₁
Solve for V-
R₁V_out+R₂V_A = V-
      R₂+R₁
Which is the same as V+, so
R₁V_out+R₂V_A = V_B R₂
      R₂+R₁             R₁+R₂
Which can be solved for V_out
                                                                                         V_out = R₂/R₁ (V_B-V_A)

Design Notes: Op-amp Stability Analysis in SPICE

Op-amp stability analysis involves getting an DC operating point with the amplifier running normally then opening the feedback loop and getting small-signal AC information. This can be done by using a very large inductor to pass DC and block AC and a very large capacitor to pass AC and block DC, but I've only seen it done that way once (and that was just last week, go figure).
HSPICE and many of its relatives, including SmartSpice, have the ability for a resistor to have a different value depening on if the simulation is DC (like an operating point analysis) or AC. The schematic below shows a simple way to accomplish this. This even works with Cadence's analogLib res, but I have not gotten it work with Cadence's analogLib resBias. But if you have access to Cadence you probably want to use the stb analysis and skip all of this. 

The listing shown here is for Silvaco's SmartSpice. HSPICE isn't that different. 


.option

+ post=1
+ measout
+ nomod
+ unwrap=1
+ usedegrees
+ savemfiles=1
+ savemodelslog
+ ingold=1
.temp 25
.AC DEC 50 10 10000meg
.LET AC mGAIN = vdb(out)-vdb(pos)
.LET AC mGAINM = -(vdb(out)-vdb(pos))
.LET AC mPHASE = vp(out, pos)
.LET AC mPHASEM = 180+(vp(out, pos))
.MEAS AC mGANMRG FIND mGAINM WHEN mPHASE=-180
.MEAS AC mPHSMRG FIND mPHASEM WHEN mGAIN=0
.MEAS AC mUGF WHEN mGAIN=0


There have been some some health problems in my family, so this was just a quick note to keep this blog weekly. I expect to have assembly and operation notes on the LED drivers next week. 

Bruce McLaren

IC Design Interview 2: Miller Op-Amp

I sprinkle Miller op-amps around my designs. They are useful to drive low capacitance internal signal lines. They make fine buffer amplifiers. Many IC designers seem to be dead set against them, but I think they do have their place. Even those engineers who would never use a Miller op-amp and spend a fair amount of time during the interview explaining why no one should ever use them will expect good knowledge of the structure, theory, and practice of design and use of the lowly Miller op-amp.

These are my interview notes so they only cover the things I tend to forget, i.e., the equations.

I am hoping to have interior trailer lights ready for next week.

Bruce McLaren