### Author Topic: Measuring transconductance of any circuit  (Read 2212 times)

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#### promach

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##### Re: Measuring transconductance of any circuit
« Reply #25 on: December 17, 2018, 12:44:56 pm »
I can now obtain gm values of each individual mosfets using built-in ngspice command

1) For your test circuit, I am still a little bit confused on the need for connecting the output node of DUT circuit to the E1/E2 input terminal ? since the other input terminal is a fixed voltage source, this looks like a voltage comparator to me ?
2) What is the purpose of E1/E2 in transconductance measurement ? simply acting as a voltage gain ?
3) And is there really a need for the current source (which also connects with the output node of DUT) in the test circuit ?

In other words, it "seems" to me that the test circuit connection with the DUT does not correctly reflect the test measurement definition of transconductance which is delta[Iout]/delta[Vin]

Feel free to correct my statements though.
« Last Edit: December 17, 2018, 01:18:58 pm by promach »

#### Wimberleytech

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##### Re: Measuring transconductance of any circuit
« Reply #26 on: December 17, 2018, 01:51:49 pm »
Lets use this circuit for our discussion.

The concept behind this test scheme (which can be broadly applied to other types of circuits) is as follows.

Using feedback, I force a known output current from the Gm stage.  I force Iout to be equal to I3.  I also force the output voltage to be equal to V1 using the opamp, U1.  The output of the opamp drives the input to the Gm stage to whatever voltage needed to achieve the current balance with I3.

Once I have this established, I vary the input current I3...I have Delta I.  This Delta I is the output of the Gm stage because the feedback requires it.  The input voltage to the Gm stage needed to achieve the Delta I is simply Delta V.  There you have it. dI/dV

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#### promach

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##### Re: Measuring transconductance of any circuit
« Reply #27 on: December 17, 2018, 02:13:39 pm »
Quote
Using feedback, I force a known output current from the Gm stage.  I force Iout to be equal to I3.

Is the current source I3 placed in reverse direction ? the arrow should point to GND since all the current Iout from DUT flows towards I3 and the ideal opamp U1 input terminal does not take any input current

Why is the current source I3 using sine pattern ?
This could easily complicate the computation since sine might involve some 'rms' or 'avg' syntax during Gm calculation ? or are there any specific reasons for choosing sine pattern other than "it works" ?

#### Wimberleytech

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##### Re: Measuring transconductance of any circuit
« Reply #28 on: December 17, 2018, 02:45:41 pm »
Quote
Using feedback, I force a known output current from the Gm stage.  I force Iout to be equal to I3.

Is the current source I3 placed in reverse direction ? the arrow should point to GND since all the current Iout from DUT flows towards I3 and the ideal opamp U1 input terminal does not take any input current
The Gm stage is biased to an output voltage of 2.5 volts.  Thus the current can point either way.  In fact, it is oscillating plus and minus.
Yes, opamp takes no current.
Quote
Why is the current source I3 using sine pattern ?
This could easily complicate the computation since sine might involve some 'rms' or 'avg' syntax during Gm calculation ? or are there any specific reasons for choosing sine pattern other than "it works" ?
Pick any pattern you want.  Sine is convenient.  No issue with RMS.  Just use peak value or peak-to-peak value.  Do the same for the opamp output for consistency.

#### promach

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##### Re: Measuring transconductance of any circuit
« Reply #29 on: December 23, 2018, 10:22:29 am »
Please see the following test circuit (for CMOS inverter) that shows gm_pmos + gm_nmos = overall Gm

Quote
[phung@archlinux frequency_trap]\$ ngspice test_CMOS_Inverter.net
******
** ngspice-29 : Circuit level simulation program
** The U. C. Berkeley CAD Group
** Copyright 1985-1994, Regents of the University of California.
** Creation Date: Sun Nov 11 02:18:13 UTC 2018
******

Circuit: * gnetlist -l ../.. -g spice-noqsi -o test_cmos_inverter.net test_cmos_inverter.sch

Doing analysis at TEMP = 25.000000 and TNOM = 27.000000

No. of Data Rows : 100
* gnetlist -l ../.. -g spice-noqsi -o test_cmos_inverter.net test_cmos_inverter.sch
AC Analysis  Sun Dec 23 18:14:37  2018
--------------------------------------------------------------------------------
Index   frequency       @m.x1.m1[gm]
--------------------------------------------------------------------------------
0   1.000000e+00   2.080796e-03,   0.000000e+00
1   1.010101e+08   2.080796e-03,   0.000000e+00
2   2.020202e+08   2.080796e-03,   0.000000e+00
3   3.030303e+08   2.080796e-03,   0.000000e+00
4   4.040404e+08   2.080796e-03,   0.000000e+00
5   5.050505e+08   2.080796e-03,   0.000000e+00
6   6.060606e+08   2.080796e-03,   0.000000e+00
7   7.070707e+08   2.080796e-03,   0.000000e+00
8   8.080808e+08   2.080796e-03,   0.000000e+00
9   9.090909e+08   2.080796e-03,   0.000000e+00
10   1.010101e+09   2.080796e-03,   0.000000e+00
11   1.111111e+09   2.080796e-03,   0.000000e+00
12   1.212121e+09   2.080796e-03,   0.000000e+00
13   1.313131e+09   2.080796e-03,   0.000000e+00
14   1.414141e+09   2.080796e-03,   0.000000e+00
15   1.515152e+09   2.080796e-03,   0.000000e+00
16   1.616162e+09   2.080796e-03,   0.000000e+00
17   1.717172e+09   2.080796e-03,   0.000000e+00
18   1.818182e+09   2.080796e-03,   0.000000e+00
19   1.919192e+09   2.080796e-03,   0.000000e+00
20   2.020202e+09   2.080796e-03,   0.000000e+00
21   2.121212e+09   2.080796e-03,   0.000000e+00
22   2.222222e+09   2.080796e-03,   0.000000e+00
23   2.323232e+09   2.080796e-03,   0.000000e+00
24   2.424242e+09   2.080796e-03,   0.000000e+00
25   2.525253e+09   2.080796e-03,   0.000000e+00
26   2.626263e+09   2.080796e-03,   0.000000e+00
27   2.727273e+09   2.080796e-03,   0.000000e+00
28   2.828283e+09   2.080796e-03,   0.000000e+00
29   2.929293e+09   2.080796e-03,   0.000000e+00
30   3.030303e+09   2.080796e-03,   0.000000e+00
31   3.131313e+09   2.080796e-03,   0.000000e+00
32   3.232323e+09   2.080796e-03,   0.000000e+00
33   3.333333e+09   2.080796e-03,   0.000000e+00
34   3.434343e+09   2.080796e-03,   0.000000e+00
35   3.535354e+09   2.080796e-03,   0.000000e+00
36   3.636364e+09   2.080796e-03,   0.000000e+00
37   3.737374e+09   2.080796e-03,   0.000000e+00
38   3.838384e+09   2.080796e-03,   0.000000e+00
39   3.939394e+09   2.080796e-03,   0.000000e+00
40   4.040404e+09   2.080796e-03,   0.000000e+00
41   4.141414e+09   2.080796e-03,   0.000000e+00
42   4.242424e+09   2.080796e-03,   0.000000e+00
43   4.343434e+09   2.080796e-03,   0.000000e+00
44   4.444444e+09   2.080796e-03,   0.000000e+00
45   4.545455e+09   2.080796e-03,   0.000000e+00
46   4.646465e+09   2.080796e-03,   0.000000e+00
47   4.747475e+09   2.080796e-03,   0.000000e+00
48   4.848485e+09   2.080796e-03,   0.000000e+00
49   4.949495e+09   2.080796e-03,   0.000000e+00
50   5.050505e+09   2.080796e-03,   0.000000e+00
51   5.151515e+09   2.080796e-03,   0.000000e+00
52   5.252525e+09   2.080796e-03,   0.000000e+00
53   5.353535e+09   2.080796e-03,   0.000000e+00
54   5.454545e+09   2.080796e-03,   0.000000e+00

Index   frequency       @m.x1.m1[gm]
--------------------------------------------------------------------------------
55   5.555556e+09   2.080796e-03,   0.000000e+00
56   5.656566e+09   2.080796e-03,   0.000000e+00
57   5.757576e+09   2.080796e-03,   0.000000e+00
58   5.858586e+09   2.080796e-03,   0.000000e+00
59   5.959596e+09   2.080796e-03,   0.000000e+00
60   6.060606e+09   2.080796e-03,   0.000000e+00
61   6.161616e+09   2.080796e-03,   0.000000e+00
62   6.262626e+09   2.080796e-03,   0.000000e+00
63   6.363636e+09   2.080796e-03,   0.000000e+00
64   6.464646e+09   2.080796e-03,   0.000000e+00
65   6.565657e+09   2.080796e-03,   0.000000e+00
66   6.666667e+09   2.080796e-03,   0.000000e+00
67   6.767677e+09   2.080796e-03,   0.000000e+00
68   6.868687e+09   2.080796e-03,   0.000000e+00
69   6.969697e+09   2.080796e-03,   0.000000e+00
70   7.070707e+09   2.080796e-03,   0.000000e+00
71   7.171717e+09   2.080796e-03,   0.000000e+00
72   7.272727e+09   2.080796e-03,   0.000000e+00
73   7.373737e+09   2.080796e-03,   0.000000e+00
74   7.474747e+09   2.080796e-03,   0.000000e+00
75   7.575758e+09   2.080796e-03,   0.000000e+00
76   7.676768e+09   2.080796e-03,   0.000000e+00
77   7.777778e+09   2.080796e-03,   0.000000e+00
78   7.878788e+09   2.080796e-03,   0.000000e+00
79   7.979798e+09   2.080796e-03,   0.000000e+00
80   8.080808e+09   2.080796e-03,   0.000000e+00
81   8.181818e+09   2.080796e-03,   0.000000e+00
82   8.282828e+09   2.080796e-03,   0.000000e+00
83   8.383838e+09   2.080796e-03,   0.000000e+00
84   8.484848e+09   2.080796e-03,   0.000000e+00
85   8.585859e+09   2.080796e-03,   0.000000e+00
86   8.686869e+09   2.080796e-03,   0.000000e+00
87   8.787879e+09   2.080796e-03,   0.000000e+00
88   8.888889e+09   2.080796e-03,   0.000000e+00
89   8.989899e+09   2.080796e-03,   0.000000e+00
90   9.090909e+09   2.080796e-03,   0.000000e+00
91   9.191919e+09   2.080796e-03,   0.000000e+00
92   9.292929e+09   2.080796e-03,   0.000000e+00
93   9.393939e+09   2.080796e-03,   0.000000e+00
94   9.494949e+09   2.080796e-03,   0.000000e+00
95   9.595960e+09   2.080796e-03,   0.000000e+00
96   9.696970e+09   2.080796e-03,   0.000000e+00
97   9.797980e+09   2.080796e-03,   0.000000e+00
98   9.898990e+09   2.080796e-03,   0.000000e+00
99   1.000000e+10   2.080796e-03,   0.000000e+00
* gnetlist -l ../.. -g spice-noqsi -o test_cmos_inverter.net test_cmos_inverter.sch
AC Analysis  Sun Dec 23 18:14:37  2018
--------------------------------------------------------------------------------
Index   frequency       @m.x1.m4[gm]
--------------------------------------------------------------------------------
0   1.000000e+00   1.145951e-03,   0.000000e+00
1   1.010101e+08   1.145951e-03,   0.000000e+00
2   2.020202e+08   1.145951e-03,   0.000000e+00
3   3.030303e+08   1.145951e-03,   0.000000e+00
4   4.040404e+08   1.145951e-03,   0.000000e+00
5   5.050505e+08   1.145951e-03,   0.000000e+00
6   6.060606e+08   1.145951e-03,   0.000000e+00
7   7.070707e+08   1.145951e-03,   0.000000e+00
8   8.080808e+08   1.145951e-03,   0.000000e+00
9   9.090909e+08   1.145951e-03,   0.000000e+00
10   1.010101e+09   1.145951e-03,   0.000000e+00
11   1.111111e+09   1.145951e-03,   0.000000e+00
12   1.212121e+09   1.145951e-03,   0.000000e+00
13   1.313131e+09   1.145951e-03,   0.000000e+00
14   1.414141e+09   1.145951e-03,   0.000000e+00
15   1.515152e+09   1.145951e-03,   0.000000e+00
16   1.616162e+09   1.145951e-03,   0.000000e+00
17   1.717172e+09   1.145951e-03,   0.000000e+00
18   1.818182e+09   1.145951e-03,   0.000000e+00
19   1.919192e+09   1.145951e-03,   0.000000e+00
20   2.020202e+09   1.145951e-03,   0.000000e+00
21   2.121212e+09   1.145951e-03,   0.000000e+00
22   2.222222e+09   1.145951e-03,   0.000000e+00
23   2.323232e+09   1.145951e-03,   0.000000e+00
24   2.424242e+09   1.145951e-03,   0.000000e+00
25   2.525253e+09   1.145951e-03,   0.000000e+00
26   2.626263e+09   1.145951e-03,   0.000000e+00
27   2.727273e+09   1.145951e-03,   0.000000e+00
28   2.828283e+09   1.145951e-03,   0.000000e+00
29   2.929293e+09   1.145951e-03,   0.000000e+00
30   3.030303e+09   1.145951e-03,   0.000000e+00
31   3.131313e+09   1.145951e-03,   0.000000e+00
32   3.232323e+09   1.145951e-03,   0.000000e+00
33   3.333333e+09   1.145951e-03,   0.000000e+00
34   3.434343e+09   1.145951e-03,   0.000000e+00
35   3.535354e+09   1.145951e-03,   0.000000e+00
36   3.636364e+09   1.145951e-03,   0.000000e+00
37   3.737374e+09   1.145951e-03,   0.000000e+00
38   3.838384e+09   1.145951e-03,   0.000000e+00
39   3.939394e+09   1.145951e-03,   0.000000e+00
40   4.040404e+09   1.145951e-03,   0.000000e+00
41   4.141414e+09   1.145951e-03,   0.000000e+00
42   4.242424e+09   1.145951e-03,   0.000000e+00
43   4.343434e+09   1.145951e-03,   0.000000e+00
44   4.444444e+09   1.145951e-03,   0.000000e+00
45   4.545455e+09   1.145951e-03,   0.000000e+00
46   4.646465e+09   1.145951e-03,   0.000000e+00
47   4.747475e+09   1.145951e-03,   0.000000e+00
48   4.848485e+09   1.145951e-03,   0.000000e+00
49   4.949495e+09   1.145951e-03,   0.000000e+00
50   5.050505e+09   1.145951e-03,   0.000000e+00
51   5.151515e+09   1.145951e-03,   0.000000e+00
52   5.252525e+09   1.145951e-03,   0.000000e+00
53   5.353535e+09   1.145951e-03,   0.000000e+00
54   5.454545e+09   1.145951e-03,   0.000000e+00

Index   frequency       @m.x1.m4[gm]
--------------------------------------------------------------------------------
55   5.555556e+09   1.145951e-03,   0.000000e+00
56   5.656566e+09   1.145951e-03,   0.000000e+00
57   5.757576e+09   1.145951e-03,   0.000000e+00
58   5.858586e+09   1.145951e-03,   0.000000e+00
59   5.959596e+09   1.145951e-03,   0.000000e+00
60   6.060606e+09   1.145951e-03,   0.000000e+00
61   6.161616e+09   1.145951e-03,   0.000000e+00
62   6.262626e+09   1.145951e-03,   0.000000e+00
63   6.363636e+09   1.145951e-03,   0.000000e+00
64   6.464646e+09   1.145951e-03,   0.000000e+00
65   6.565657e+09   1.145951e-03,   0.000000e+00
66   6.666667e+09   1.145951e-03,   0.000000e+00
67   6.767677e+09   1.145951e-03,   0.000000e+00
68   6.868687e+09   1.145951e-03,   0.000000e+00
69   6.969697e+09   1.145951e-03,   0.000000e+00
70   7.070707e+09   1.145951e-03,   0.000000e+00
71   7.171717e+09   1.145951e-03,   0.000000e+00
72   7.272727e+09   1.145951e-03,   0.000000e+00
73   7.373737e+09   1.145951e-03,   0.000000e+00
74   7.474747e+09   1.145951e-03,   0.000000e+00
75   7.575758e+09   1.145951e-03,   0.000000e+00
76   7.676768e+09   1.145951e-03,   0.000000e+00
77   7.777778e+09   1.145951e-03,   0.000000e+00
78   7.878788e+09   1.145951e-03,   0.000000e+00
79   7.979798e+09   1.145951e-03,   0.000000e+00
80   8.080808e+09   1.145951e-03,   0.000000e+00
81   8.181818e+09   1.145951e-03,   0.000000e+00
82   8.282828e+09   1.145951e-03,   0.000000e+00
83   8.383838e+09   1.145951e-03,   0.000000e+00
84   8.484848e+09   1.145951e-03,   0.000000e+00
85   8.585859e+09   1.145951e-03,   0.000000e+00
86   8.686869e+09   1.145951e-03,   0.000000e+00
87   8.787879e+09   1.145951e-03,   0.000000e+00
88   8.888889e+09   1.145951e-03,   0.000000e+00
89   8.989899e+09   1.145951e-03,   0.000000e+00
90   9.090909e+09   1.145951e-03,   0.000000e+00
91   9.191919e+09   1.145951e-03,   0.000000e+00
92   9.292929e+09   1.145951e-03,   0.000000e+00
93   9.393939e+09   1.145951e-03,   0.000000e+00
94   9.494949e+09   1.145951e-03,   0.000000e+00
95   9.595960e+09   1.145951e-03,   0.000000e+00
96   9.696970e+09   1.145951e-03,   0.000000e+00
97   9.797980e+09   1.145951e-03,   0.000000e+00
98   9.898990e+09   1.145951e-03,   0.000000e+00
99   1.000000e+10   1.145951e-03,   0.000000e+00
* gnetlist -l ../.. -g spice-noqsi -o test_cmos_inverter.net test_cmos_inverter.sch
AC Analysis  Sun Dec 23 18:14:37  2018
--------------------------------------------------------------------------------
Index   frequency       gm
--------------------------------------------------------------------------------
0   1.000000e+00   6.338799e-12,   3.979701e-09
1   1.010101e+08   3.226539e-03,   2.589921e-05
2   2.020202e+08   3.226695e-03,   1.295023e-05
3   3.030303e+08   3.226724e-03,   8.633565e-06
4   4.040404e+08   3.226734e-03,   6.475194e-06
5   5.050505e+08   3.226738e-03,   5.180163e-06
6   6.060606e+08   3.226741e-03,   4.316806e-06
7   7.070707e+08   3.226742e-03,   3.700121e-06
8   8.080808e+08   3.226743e-03,   3.237607e-06
9   9.090909e+08   3.226744e-03,   2.877873e-06
10   1.010101e+09   3.226745e-03,   2.590086e-06
11   1.111111e+09   3.226745e-03,   2.354624e-06
12   1.212121e+09   3.226745e-03,   2.158406e-06
13   1.313131e+09   3.226745e-03,   1.992375e-06
14   1.414141e+09   3.226746e-03,   1.850062e-06
15   1.515152e+09   3.226746e-03,   1.726725e-06
16   1.616162e+09   3.226746e-03,   1.618805e-06
17   1.717172e+09   3.226746e-03,   1.523581e-06
18   1.818182e+09   3.226746e-03,   1.438938e-06
19   1.919192e+09   3.226746e-03,   1.363204e-06
20   2.020202e+09   3.226746e-03,   1.295044e-06
21   2.121212e+09   3.226746e-03,   1.233375e-06
22   2.222222e+09   3.226746e-03,   1.177313e-06
23   2.323232e+09   3.226746e-03,   1.126125e-06
24   2.424242e+09   3.226746e-03,   1.079203e-06
25   2.525253e+09   3.226746e-03,   1.036035e-06
26   2.626263e+09   3.226746e-03,   9.961877e-07
27   2.727273e+09   3.226746e-03,   9.592918e-07
28   2.828283e+09   3.226746e-03,   9.250314e-07
29   2.929293e+09   3.226746e-03,   8.931338e-07
30   3.030303e+09   3.226746e-03,   8.633627e-07
31   3.131313e+09   3.226747e-03,   8.355122e-07
32   3.232323e+09   3.226747e-03,   8.094025e-07
33   3.333333e+09   3.226747e-03,   7.848751e-07
34   3.434343e+09   3.226747e-03,   7.617906e-07
35   3.535354e+09   3.226747e-03,   7.400251e-07
36   3.636364e+09   3.226747e-03,   7.194689e-07
37   3.737374e+09   3.226747e-03,   7.000238e-07
38   3.838384e+09   3.226747e-03,   6.816021e-07
39   3.939394e+09   3.226747e-03,   6.641251e-07
40   4.040404e+09   3.226747e-03,   6.475220e-07
41   4.141414e+09   3.226747e-03,   6.317288e-07
42   4.242424e+09   3.226747e-03,   6.166876e-07
43   4.343434e+09   3.226747e-03,   6.023461e-07
44   4.444444e+09   3.226747e-03,   5.886564e-07
45   4.545455e+09   3.226747e-03,   5.755751e-07
46   4.646465e+09   3.226747e-03,   5.630626e-07
47   4.747475e+09   3.226747e-03,   5.510826e-07
48   4.848485e+09   3.226747e-03,   5.396017e-07
49   4.949495e+09   3.226747e-03,   5.285894e-07
50   5.050505e+09   3.226747e-03,   5.180176e-07
51   5.151515e+09   3.226747e-03,   5.078604e-07
52   5.252525e+09   3.226747e-03,   4.980939e-07
53   5.353535e+09   3.226747e-03,   4.886959e-07
54   5.454545e+09   3.226747e-03,   4.796459e-07

Index   frequency       gm
--------------------------------------------------------------------------------
55   5.555556e+09   3.226747e-03,   4.709251e-07
56   5.656566e+09   3.226747e-03,   4.625157e-07
57   5.757576e+09   3.226747e-03,   4.544014e-07
58   5.858586e+09   3.226747e-03,   4.465669e-07
59   5.959596e+09   3.226747e-03,   4.389980e-07
60   6.060606e+09   3.226747e-03,   4.316813e-07
61   6.161616e+09   3.226747e-03,   4.246046e-07
62   6.262626e+09   3.226747e-03,   4.177561e-07
63   6.363636e+09   3.226747e-03,   4.111251e-07
64   6.464646e+09   3.226747e-03,   4.047013e-07
65   6.565657e+09   3.226747e-03,   3.984751e-07
66   6.666667e+09   3.226747e-03,   3.924376e-07
67   6.767677e+09   3.226747e-03,   3.865803e-07
68   6.868687e+09   3.226747e-03,   3.808953e-07
69   6.969697e+09   3.226747e-03,   3.753751e-07
70   7.070707e+09   3.226747e-03,   3.700126e-07
71   7.171717e+09   3.226747e-03,   3.648011e-07
72   7.272727e+09   3.226747e-03,   3.597345e-07
73   7.373737e+09   3.226747e-03,   3.548066e-07
74   7.474747e+09   3.226747e-03,   3.500119e-07
75   7.575758e+09   3.226747e-03,   3.453451e-07
76   7.676768e+09   3.226747e-03,   3.408011e-07
77   7.777778e+09   3.226747e-03,   3.363751e-07
78   7.878788e+09   3.226747e-03,   3.320626e-07
79   7.979798e+09   3.226747e-03,   3.278593e-07
80   8.080808e+09   3.226747e-03,   3.237610e-07
81   8.181818e+09   3.226747e-03,   3.197640e-07
82   8.282828e+09   3.226747e-03,   3.158644e-07
83   8.383838e+09   3.226747e-03,   3.120588e-07
84   8.484848e+09   3.226747e-03,   3.083438e-07
85   8.585859e+09   3.226747e-03,   3.047162e-07
86   8.686869e+09   3.226747e-03,   3.011730e-07
87   8.787879e+09   3.226747e-03,   2.977113e-07
88   8.888889e+09   3.226747e-03,   2.943282e-07
89   8.989899e+09   3.226747e-03,   2.910211e-07
90   9.090909e+09   3.226747e-03,   2.877876e-07
91   9.191919e+09   3.226747e-03,   2.846251e-07
92   9.292929e+09   3.226747e-03,   2.815313e-07
93   9.393939e+09   3.226747e-03,   2.785041e-07
94   9.494949e+09   3.226747e-03,   2.755413e-07
95   9.595960e+09   3.226747e-03,   2.726409e-07
96   9.696970e+09   3.226747e-03,   2.698008e-07
97   9.797980e+09   3.226747e-03,   2.670194e-07
98   9.898990e+09   3.226747e-03,   2.642947e-07
99   1.000000e+10   3.226747e-03,   2.616251e-07
ngspice 1219 ->

Smf