Closed-loop AM demodulator 

*
* POWER SUPPLIES
*
vcc       vcc       0        12
vcm       cm        0         6

*
* INPUTS
*
vin       in        cm        ac       1         am       sa oc 4k 455k
.param    index=0.5           * index = modulation index
.param    amp=100m            * amp = p-p amplitude of modulated waveform
.param    oc='(1/index)'      * oc = "offset constant" in hspice
.param    sa='index*amp'      * sa = "signal amplitude" constant in hspice

*
* CIRCUIT
*
* To simplify the simulation, the input is biased at mid-supply
* and capacitor C31 is not included in the simulation

xq1       vcc       in        e1        t2n3904
xq2       c2        out       e1        t2n3904
xq3       vcc       c2        out       t2n3904
xq4       e1        bias      0         t2n3904
xq5       bias      bias      0         t2n3904
r31       vcc       in        1k
r32       in        0         1k
r33       vcc       bias      20k
r34       vcc       c2        10k
r35       out       0         r35
c32       out       0         c32

.param    r35=
.param    c32=


*
* ANALYSIS
*
.options  nomod post=2 acout=0
.op
.tran 100n 1000u
.print tran v(out)
.four 4k v(out)
* The Fourier analysis command can be used to see the output distortion,
* which may be large for some values of R35 and C32.

.end