Variable Gain Preamplifier using dc control voltage

[m_t_c]

Hi all

I am trying to simulate input stage of a variable gain preamplifier for acoustic hydrophone operating in frequency range of 90KHz to 110KHz. It is transistor based preamplifier. The gain of preamplifier is controlled through dc control voltage. I have attached the Orcad Capture Schematic snapshot.

The problem is that whatever dc control i set, output is still zero!

Any help in this regard is much appreciated.

 

[danadakk]

Run a bias sim first to see if you have the transistors biased properly for starters.

Probe Q01 collector, signal present there ?

 

[D.A.(Tony)Stewart]

The circuit works but could be improved greatly if you do not want low gain, low gain sensitivity, and low input impedance.

Do you have any more design specs?

 

[m_t_c]

Run a bias sim first to see if you have the transistors biased properly for starters.

Probe Q01 collector, signal present there ?

--- Updated Apr 28, 2025 ---

Do you have any more design specs?

The specifications of complete preamplifier are attached.

 

[FvM]

My estimation gives about 6 dB gain for the present operation point (0.66 mA). There will be definitely signal output with 1 mV input.

I don't understand what 80 dB gain spec refers to, shown amplifier stage can achieve about 24 - 25 dB.

 

[dick_freebird]

Are you against using an IC product? Not sure the old CA3080 is still available, but more gain for less effort and surprises in an integrated VGA.

--- Updated Apr 28, 2025 ---

A=80dB (10000) and 100kHz makes 1GHz GBWP required....

 

[danadakk]

Cant believe you can still get CA3080, sure enough Digikey has them.

Another alternative would be LM13700.

 

[m_t_c]

gives about 6 dB gain for the present operation point (0.66 mA). There will be definitely signal output with 1 mV input.

I don't understand what 80 dB gain spec refers to, shown amplifier stage can achieve about 24 - 25 dB.I have shared only first stage of complete preamplifier connected with acoustic sensor, there are 02 more stages like it with total gain variation of 80 dB. Problem is that whatever i set of Vc value, output at transformer secondary remains zero

My estimation gives about 6 dB gain for the present operation point (0.66 mA). There will be definitely signal output with 1 mV input.

I don't understand what 80 dB gain spec refers to, shown amplifier stage can achieve about 24 - 25 dB.

I have shared only first stage of complete preamplifier connected with acoustic sensor, there are 02 more stages like it with total gain variation of 80 dB. Problem is that whatever i set of Vc value, output at transformer secondary remains zero

 

[FvM]

Problem is that whatever i set of Vc value, output at transformer secondary remains zero

Unlikely. How do you measure output? Should be transient analysis.

 

[m_t_c]

Unlikely. How do you measure output? Should be transient analysis.

I am doing Time Domain (Transient) Analysis with 'Run to time' 1ms

 

[dick_freebird]

Just a thought, as you're looking at low input signal,
make sure your trtol (or whatever it's called in your
simulator) is well below input signal amplitude and
any other point in the signal path. Else signal can
be "swallowed up" by error tolerance. This is also a
thing, for XO simulations. Might check other tolerances
as well (chgtol, vntol, ...). Stay away from TRAP method
and its icky sawtooth residue which never damps out.

 

[FvM]

With default PSpice simulation parameters, there will be output signal. Something is wrong with the setup that can't be recognized from schematic. E.g. K1 not actually coupling L3 and L4.

 

[D.A.(Tony)Stewart]

I have shared only first stage of complete preamplifier connected with acoustic sensor, there are 02 more stages like it with total gain variation of 80 dB. Problem is that whatever i set of Vc value, output at transformer secondary remains zero

The specs are unusually defined.

Reconsider how to define them following AM radio with IF amplifiers specs and designs.
Sensitivity
BW -3dB, max amplitude ripple, max group delay ripple
Band reject f, dB
AGC range, attack, decay time
Conversion gain.
carrier out level
demod. out level.

Common IC's TA7642 or MK484

 

[m_t_c]

With default PSpice simulation parameters, there will be output signal. Something is wrong with the setup that can't be recognized from schematic. E.g. K1 not actually coupling L3 and L4.

Thanks very much for help. I have problem in simulating the transformer in Orcad Capture as i am trying to simulate filter circuit connected with Q01 collector standalone and have following errors!
"ERROR -- Node N27120 is floating
ERROR -- Node N27066 is floating
ERROR -- Voltage source and/or inductor loop involving V_V1
You may break the loop by adding a series resistance"
Have you any experience with transfrmers simulations in Orcad Pspice?

 

[kd502]

 

[FvM]

Ltspice inductor has a default series resistance, PSpice Lbreak component apparently not.

 

[dick_freebird]

Thanks very much for help. I have problem in simulating the transformer in Orcad Capture as i am trying to simulate filter circuit connected with Q01 collector standalone and have following errors!
"ERROR -- Node N27120 is floating
ERROR -- Node N27066 is floating
ERROR -- Voltage source and/or inductor loop involving V_V1
You may break the loop by adding a series resistance"
Have you any experience with transfrmers simulations in Orcad Pspice?

I have none but since this is a schematic based tool,
check the device properties for series resistance
values (may default to zero) on sources and inductors.

Similarly the "k" element probably has arg fields for
pair or higher order coupling, for you to property-edit
(or if capture tool is well integrated, maybe just pick).

 

[D.A.(Tony)Stewart]

Are you using this?

Then use differential reading for output or ground it with a 1 Gig R.

But this design offers no noise filtering for a front end and very little gain.

 

[D.A.(Tony)Stewart]

Do you really need 80 dB AGC range? and 50 Ohm input? hydrophones are not low impedance and should be matched or unloaded.
What kind of hydrophone are you using ? active or passive?

Gain dispersion is used in power vs wavelength not for ultrasound voltage. You already defined BW -3dB as >= 22 kHz.
What do mean by dispersion here? from 89 to 111 kHz?
Where are the AGC attack and decay specs?
How can you have Vs= 100 mV with 80 dB gain?

Pls. answer.

 

[FvM]

The original simulation circuit uses coupled Lbreak rather than transformer model which is basically o.k. The error in post #14 circuit is specifically caused by driving an inductor with zero series resistance by a voltage source. Post #15 has already shown that a current source would be the appropriate equivalent to original amplifier circuit. Post #14 adresses a pure simulation problem and isn't related to original question.

It couldn't be yet clarified why post #1 simulation has supposedly no signal output.