OTA (operational transconductance amplifier) code model as a
reference implementation that exercises the programmatic noise interface.
This commit is contained in:
Seth Hillbrand
Holger Vogt
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/*
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================================================================================
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FILE ota/cfunc.mod
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Public Domain
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AUTHORS
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20 Mar 2026 Seth Hillbrand
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SUMMARY
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This file contains the model-specific routines for the OTA
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(Operational Transconductance Amplifier) code model.
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DC/TRAN: Vout as current = gm * (Vp - Vn)
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AC: Complex gain = gm
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NOISE: Programmatic noise with en, in_noise, enk, ink, incm, incmk.
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Parameter naming follows LTSPICE convention for OTA compatibility.
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================================================================================
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*/
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#include <stdlib.h>
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void cm_ota(ARGS)
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{
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double gm_val = PARAM(gm);
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double rout = PARAM(rout);
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double rin = PARAM(rin);
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Mif_Complex_t ac_gain;
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if (ANALYSIS == NOISE) {
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/* Register noise sources on every call (N_OPEN and N_CALC).
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* During N_OPEN: registers and returns index.
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* During N_CALC: returns same sequential index. */
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int src_en_w = cm_noise_add_source("en_white", 1, 0, MIF_NOISE_CURRENT);
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int src_en_f = cm_noise_add_source("en_flicker", 1, 0, MIF_NOISE_CURRENT);
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int src_in_w = cm_noise_add_source("in_white", 0, 0, MIF_NOISE_CURRENT);
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int src_in_f = cm_noise_add_source("in_flicker", 0, 0, MIF_NOISE_CURRENT);
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/* Common-mode current noise: independent sources from each input pin to ground */
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int src_icm_pw = cm_noise_add_source("incm_p_white", 0, 0, MIF_NOISE_CURRENT_POS);
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int src_icm_pf = cm_noise_add_source("incm_p_flicker", 0, 0, MIF_NOISE_CURRENT_POS);
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int src_icm_nw = cm_noise_add_source("incm_n_white", 0, 0, MIF_NOISE_CURRENT_NEG);
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int src_icm_nf = cm_noise_add_source("incm_n_flicker", 0, 0, MIF_NOISE_CURRENT_NEG);
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if (!mif_private->noise->registering) {
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double en = PARAM(en);
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double in_n = PARAM(in_noise);
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double enk_val = PARAM(enk);
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double ink_val = PARAM(ink);
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double incm = PARAM(incm);
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double incmk_val = PARAM(incmk);
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double f = NOISE_FREQ;
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/* en referred to output as current noise: (en * gm)^2 A^2/Hz */
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NOISE_DENSITY(src_en_w) = en * en * gm_val * gm_val;
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NOISE_DENSITY(src_en_f) = (enk_val > 0 && f > 0) ?
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en * en * gm_val * gm_val * enk_val / f : 0.0;
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/* in as differential current noise at input: in^2 A^2/Hz */
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NOISE_DENSITY(src_in_w) = in_n * in_n;
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NOISE_DENSITY(src_in_f) = (ink_val > 0 && f > 0) ?
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in_n * in_n * ink_val / f : 0.0;
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/* incm as current noise from each input pin to ground */
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NOISE_DENSITY(src_icm_pw) = incm * incm;
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NOISE_DENSITY(src_icm_pf) = (incmk_val > 0 && f > 0) ?
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incm * incm * incmk_val / f : 0.0;
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NOISE_DENSITY(src_icm_nw) = incm * incm;
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NOISE_DENSITY(src_icm_nf) = (incmk_val > 0 && f > 0) ?
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incm * incm * incmk_val / f : 0.0;
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}
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return;
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}
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if (ANALYSIS != MIF_AC) {
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double v_in = INPUT(inp);
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double v_out = INPUT(out);
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OUTPUT(out) = gm_val * v_in + v_out / rout;
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PARTIAL(out, inp) = gm_val;
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PARTIAL(out, out) = 1.0 / rout;
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OUTPUT(inp) = v_in / rin;
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PARTIAL(inp, inp) = 1.0 / rin;
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}
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else {
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Mif_Complex_t ac_rout;
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Mif_Complex_t ac_rin;
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ac_gain.real = gm_val;
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ac_gain.imag = 0.0;
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AC_GAIN(out, inp) = ac_gain;
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ac_rout.real = 1.0 / rout;
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ac_rout.imag = 0.0;
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AC_GAIN(out, out) = ac_rout;
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ac_rin.real = 1.0 / rin;
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ac_rin.imag = 0.0;
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AC_GAIN(inp, inp) = ac_rin;
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}
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}
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@ -0,0 +1,114 @@
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/*
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================================================================================
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Public Domain
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SUMMARY
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Interface specification for the OTA (Operational Transconductance Amplifier)
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code model. LTSPICE-compatible parameter naming convention.
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Noise is implemented via the programmatic API (cm_noise_add_source /
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NOISE_DENSITY). Set noise_programmatic = TRUE to enable noise analysis.
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================================================================================
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*/
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NAME_TABLE:
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C_Function_Name: cm_ota
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Spice_Model_Name: ota
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Description: "Operational Transconductance Amplifier with noise"
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PORT_TABLE:
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Port_Name: inp out
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Description: "input" "output"
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Direction: inout inout
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Default_Type: gd g
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Allowed_Types: [gd] [g]
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Vector: no no
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Vector_Bounds: - -
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Null_Allowed: no no
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PARAMETER_TABLE:
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Parameter_Name: gm rout
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Description: "transconductance" "output resistance"
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Data_Type: real real
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Default_Value: 1.0e-3 1.0e12
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Limits: [1e-15 -] [0 -]
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Vector: no no
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Vector_Bounds: - -
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Null_Allowed: yes yes
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PARAMETER_TABLE:
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Parameter_Name: rin
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Description: "input resistance"
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Data_Type: real
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Default_Value: 1.0e12
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Limits: [0 -]
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Vector: no
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Vector_Bounds: -
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Null_Allowed: yes
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PARAMETER_TABLE:
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Parameter_Name: en in_noise
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Description: "input voltage noise density V/rtHz" "input current noise density A/rtHz"
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Data_Type: real real
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Default_Value: 0.0 0.0
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Limits: [0 -] [0 -]
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Vector: no no
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Vector_Bounds: - -
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Null_Allowed: yes yes
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PARAMETER_TABLE:
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Parameter_Name: enk ink
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Description: "voltage noise 1/f corner Hz" "current noise 1/f corner Hz"
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Data_Type: real real
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Default_Value: 0.0 0.0
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Limits: [0 -] [0 -]
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Vector: no no
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Vector_Bounds: - -
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Null_Allowed: yes yes
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PARAMETER_TABLE:
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Parameter_Name: incm incmk
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Description: "CM current noise density A/rtHz" "CM current noise 1/f corner Hz"
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Data_Type: real real
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Default_Value: 0.0 0.0
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Limits: [0 -] [0 -]
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Vector: no no
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Vector_Bounds: - -
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Null_Allowed: yes yes
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PARAMETER_TABLE:
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Parameter_Name: noise_programmatic
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Description: "enable programmatic noise sources"
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Data_Type: boolean
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Default_Value: TRUE
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Limits: -
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Vector: no
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Vector_Bounds: -
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Null_Allowed: yes
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