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# Calculating gain in dB.

#### Lightium

##### Active Member
How can I calculate in LTspice the gain in decibels of my medium wave RF amplifier?

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For voltage gain: 20*log(Vout/Vin)
For power gain: 10*log(Pout/Pin)

Chek the LTspice HELP file for the "Method of Middlebrook" to get the series of .meas statements for doing this in a .tran analysis

Thank you Papabravo, That looks about right to me. Yes, I look at the help file all the time and I am still amazed at what I miss.

How can I calculate in LTspice the gain in decibels of my medium wave RF amplifier?
We were told at college, long ago, that only those who don't know what they are doing use decibels

From someone who didn't understand decibels?

We were told at college, long ago, that only those who don't know what they are doing use decibels
I guess I was under the impression that my BSE was worth something. I must have missed that lecture.
Those that can do. Those who can't teach or administrate.

Why is the resistance of R4 so high that it does not pass any signal?

How can I calculate in LTspice the gain in decibels of my medium wave RF amplifier?
This design will not amplify properly for many reason. wrong impedances.

Below is the sim with some modifications:

I disconnected the emitter capacitors since they were apparently causing the amp to oscillate from the collector inductors.
I changed the input resistance to 1Ω (obviously not much signal will go through a 1G ohm resistor).
I did an AC analysis with an input of 1V, to give the small-signal (linearized) gain in dB.

What frequency response and gain did you want?

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I was trying to make an MW RF amp with a gain of -30dB 30dB for listening to sferics at 500KHz and below. Thank you crutschow.

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I was trying to make an MW RF amp with a gain of -30dB...
Isn't that known as an attenuator?

Isn't that known as an attenuator?
All my amps are attenuators, they're atrocious.

All my amps are attenuators, they're atrocious.
So why not design proper circuits?, it's like you're just randomly connecting random values together and expecting it to work?.

If you're interested in RF, then you should have the RSGB Handbook or the ARRL handbook, or preferably both.

They don't have to be the latest versions, and probably older versions might be more use.

Will do Nigel.

I was trying to make an MW RF amp with a gain of -30dB 30dB for listening to sferics at 500KHz and below.
How will a medium wave amplifier enable you to listen to sferics below 500kHz?

JimB

JimB, I have rtl-sdr that tunes down to 100KHz.

JimB, I have rtl-sdr that tunes down to 100KHz.
So why do you need a 30dB amplifier?.

Perhaps you're not aware of how radio works? - above 30MHz or so (so VHF upwards) the limiting factor is the noise level of the front end - which is why you fit masthead amplifiers actually on the aerial, effectively moving the front end as close to the aerial as possible.

Below 30MHz the limiting factor is atmospheric noise - so amplifying doesn't really help much, as you're just amplifying the noise as well as the signal. If the frontend of the radio is particularly poor (as an SDR radio might well be) a small TUNED amplifier could well help - and frontend tuning is always going to help an SDR radio, as often they don't have any.

But you certainly don't need 30dB.

Your right Nigel. I was thinking about the front-end. I am thinking about cooling it down to 0 Celsius.

I'd find a dead frequency somewhere in the "long wave" band and build a tuned preamp for that, to feed your SDR, using it with a long wire antenna.

When looking for a suitable frequency, check at night as well as daytime, to allow for changes in propagation.

This is a full receiver schematic for around 300KHz - it has the detector straight after the TL592 RF amp stage!

Your right Nigel. I was thinking about the front-end. I am thinking about cooling it down to 0 Celsius.
There's no point for medium wave, as I've already explained the limiting factor is atmospheric noise.

I wouldn't have thought there's any point anyway, regardless of frequency, if you're only cooling to zero degrees - to improve noise at microwave frequencies requires seriously low temperatures, such as liquid helium.