# LTSpice circuit analysis questions

#### Seang

Joined May 21, 2020
14
Hi there, I am working with a HEMT schematic (design attached) on LTSpice (I only have beginner experience with it) and I would like to find out a few things about this circuit:

1. The amplification factor and dissipated power and how they depend on the operating point for the transistors
2. The bandwidth of the circuit for a range of input resistances, inductances, capacitances, etc.
3. Is there a way of modelling noise for the transistors?

I currently have a bode plot going for the schematic, and can see currents and voltages at different points of the circuit as a function of frequency. Is there also a way to include power dissipated in these plots? I can find the find the average power for various components, but how can I find the total power dissipated by the circuit? For amplification factor, what would I use as the output voltage? How do these depend on transistor conditions?

Thanks.

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#### crutschow

Joined Mar 14, 2008
25,983
how can I find the total power dissipated by the circuit?
Check the power of the V2 supply.

#### Bordodynov

Joined May 20, 2015
2,731
A little thought on the proposed scheme.
It is a bad idea to use microwave transistors to amplify such low frequencies. Strong decrease in the upper limit frequency occurs due to bypassing of transistors with capacitors of 1000 pF. Microwave transistors may have large noise flicker at low frequencies. For these 10 MHz transistors the low frequency is low. You can safely throw out the top transistor - get more gain.

#### Bordodynov

Joined May 20, 2015
2,731
See

#### Seang

Joined May 21, 2020
14
A little thought on the proposed scheme.
It is a bad idea to use microwave transistors to amplify such low frequencies. Strong decrease in the upper limit frequency occurs due to bypassing of transistors with capacitors of 1000 pF. Microwave transistors may have large noise flicker at low frequencies. For these 10 MHz transistors the low frequency is low. You can safely throw out the top transistor - get more gain.
I was playing around with your suggestion and I definitely see why we get more gain, so thanks for that. May I ask, how did you get those gain graphs like in the second photo you linked? How did you do the noise analysis?

#### Bordodynov

Joined May 20, 2015
2,731
And you know what the third picture shows.
And it shows that the original electronic circuitry is bad. It doesn't function in TRAN calculation mode. And how to do noise analysis, read HELP.
The peculiarity of my analysis is that I made an analysis of two different circuits at once. I used a SWeq switch. It has x and y parameters. When X=Y, the switch is closed.
I connect the two circuits in turn and count the noise.
When i=1, the first circuit is connected.
At i=2, the second circuit is connected.