RF Amplifier Gain Measurement in LT Spice - UK Vintage Radio Repair and Restoration Discussion Forum

Jon_G4MDC · 17th Jan 2022, 11:58 am

I wonder if anyone has been here before.

I am attempting to model a transformer coupled RF amplifier in LT Spice.

To see the Voltage Gain I put a probe on the output node and another on the input node. I see two sine waves, output one is a lot bigger than the input one so there is some hope!

To see the numerical Voltage gain I set another trace which is to display the equation Vout/Vin.

That works very well except when Vin passes through zero. Division by zero causes a huge glitch in that trace. There must be a better way.

Radio Wrangler · 17th Jan 2022, 12:35 pm

For RF gain, you need to consider source and load impedances. Voltage gain doesn't guarantee anything useful into an open circuit load. It's overall power gain which does the work (bad pun!)

David

Keith956 · 17th Jan 2022, 12:37 pm

When I measure gain / phase I run an AC analysis over the desired frequency range, with a input voltage source with AC magnitude set to 1, and then plot the output magnitude in dB:

e.g. in the netlist, set up a voltage source vin connecting to the input nodes n1/n2, then look at the plot of the output node - something like:

.ac dec 10 1meg 1g
vin n1 n2 ac 1
.plot ac vdb(vout)

GrimJosef · 17th Jan 2022, 1:03 pm

[pedant]Is it just me, or would this, strictly speaking, be a simulation rather than a measurement ?[/pedant]

(Maybe this is just a conditioned reflex having spent the better part of my working life trying to 'ground' my enthusiastic colleagues in our Theory Division by actually devising experiments to measure the fantastic things they predicted

?)

Cheers,

GJ

17th Jan 2022, 11:58 am	#1
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	RF Amplifier Gain Measurement in LT Spice I wonder if anyone has been here before. I am attempting to model a transformer coupled RF amplifier in LT Spice. To see the Voltage Gain I put a probe on the output node and another on the input node. I see two sine waves, output one is a lot bigger than the input one so there is some hope! To see the numerical Voltage gain I set another trace which is to display the equation Vout/Vin. That works very well except when Vin passes through zero. Division by zero causes a huge glitch in that trace. There must be a better way.

17th Jan 2022, 12:35 pm	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,868	Re: RF Amplifier Gain Measurement in LT Spice For RF gain, you need to consider source and load impedances. Voltage gain doesn't guarantee anything useful into an open circuit load. It's overall power gain which does the work (bad pun!) David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

17th Jan 2022, 12:37 pm	#3
Keith956 Heptode Join Date: Jun 2019 Location: Oxfordshire Posts: 738	Re: RF Amplifier Gain Measurement in LT Spice When I measure gain / phase I run an AC analysis over the desired frequency range, with a input voltage source with AC magnitude set to 1, and then plot the output magnitude in dB: e.g. in the netlist, set up a voltage source vin connecting to the input nodes n1/n2, then look at the plot of the output node - something like: .ac dec 10 1meg 1g vin n1 n2 ac 1 .plot ac vdb(vout)

17th Jan 2022, 1:03 pm	#4
GrimJosef Dekatron Join Date: Sep 2007 Location: Oxfordshire, UK. Posts: 4,311	Re: RF Amplifier Gain Measurement in LT Spice [pedant]Is it just me, or would this, strictly speaking, be a simulation rather than a measurement ?[/pedant] (Maybe this is just a conditioned reflex having spent the better part of my working life trying to 'ground' my enthusiastic colleagues in our Theory Division by actually devising experiments to measure the fantastic things they predicted ?) Cheers, GJ __________________ http://www.ampregen.com