Measuring Gain

[walters]

How do i measure Gain?

How do i measure Voltage Gain in each stage of a circuit?

How do i measure Power Gain in each stage of a circuit?

How do i measure dbs decibles in each stage of a circuit?

 

[Morgen]

Gain is the ratio of input to output.
For example, voltage out divided by voltage in equals voltage gain.

 

[ljcox]

Gain in dB is

dB = 10 log(P1/P2) where P1 is the output power and P2 is the input power, but the input and output impedances must be equal.

dB = 20 log(V1/V2) where V1 is the output voltage and V2 is the input voltage.

The 20 is because Power in a resistor is = V^2/R.

It is usual to express the voltage gain of an amplifier in dB even if the input and output impedances are not equal.

I suggest you search this forum for terms such as dB, gain, etc. as there have been other posts on this subject.

Len

 

[walters]

How do i measure it with a oscillascope how do i measure the gain?

or with a DB meter?

 

[ljcox]

You can measure the voltage with a voltmeter or an oscilloscope.

It depends on what you mean by "dB meter"

Some measure power and some measure voltage.

Len

 

[walters]

DB meter i think has a different reference voltages
should read out in DB voltages i think

 

[walters]

Its a AC volts meter only for read DBs it says

Decibels in RED
RMS volts in black

0 dbv=1volt dBV reading scale color is GREEN

0dBm= .775volts dBM reading scale colour is RED

Class= 1.5 i guess its the RMS reading scale color is BLACK


how do i use this AC meter to do BD reading gains for each in and output stages?

 

[JimB]

A decibel is the RATIO of two quantities, usually power. 10dB is a gain of 10 times the power, -10db is a power reduction of 10 times.

Decibels are sometimes used as UNITS, hence:

dBm - decibels with respect to 1mW

dBv - decibels with respect to 1 volt

dBw - decibels with respect to 1 watt.

Where a meter is measuring voltage, but is calibrated in dB, it is important to know the impedance.

Usually for audio circuits, the impedance is 600 ohm.
So for a meter where 0.775v = 0dBm:

1mW = (0.775 x 0.775)/600 Try it, do the maths!

But for RF measurements the standard impedance is usually 50 ohm.
So here the meter will be calibrated so that 0dBm = 0.223v

1mW - (0.223 x 0.223)/50


As for finding the gain of an amplifier, measure the signal level at the input and the signal level at the output.

If we decide to measure voltage, and the input is 0.06 volts and the output is 3.6volts, the gain is 3.6/0.06 = 60.

If we measure in dB, and find the input is -13dB and the output is +17dB, the gain is +17 - (-13) = +20db
To find the gain (or loss) of a stage in dB, just subtract the input from the output.

JimB

 

[David Bridgen]

How do i measure Gain?

How do i measure Voltage Gain in each stage of a circuit?

How do i measure Power Gain in each stage of a circuit?

How do i measure dbs decibles in each stage of a circuit?

etc etc etc

I see that you have posted the same, and more, questions on epanorama too.
It appears that you are seeking instant knowledge and understanding of an immense field.

I can understand your eagerness and enthusiasm but you must learn to walk before you can run.
Take your time to learn and understand the fundamental principles and basics. Without that solid base your future progres will be frustrating and limited.

As regards decibel notation you may like to return to https://www.davidbridgen.com/dbnotation.htm once you have read more.

 

[walters]

dBm - decibels with respect to 1mW

dBv - decibels with respect to 1 volt

dBw - decibels with respect to 1 watt.


Can i use the oscilloscope to measure this different db scales?

how do i do that with a oscilloscope i need measured dbm or dbv or dbw
before with a oscilloscope?

Or i can only use the AC volt meter for doing this?

 

[JimB]

An oscilloscope will only measure the peak voltage and time.

If you want dB, you need the appropriate meter, or just calculate it from voltage.

What are you really trying to do?

JimB

 

[Nigel Goodwin]

Can i use the oscilloscope to measure this different db scales?

I was told by a lecturer at college many years ago that "only people who don't know what they are talking about talk in dB's" - I've never found reason to disagree with that statement, basically dB's are 'an idiots guide to signal levels and gain' :lol:

Basically neither meters or scopes 'measure' in dB's, they measure in volts (usually!), but sometimes meters have an extra scale calibrated in dB's.

 

[ljcox]

dBm - decibels with respect to 1mW

dBv - decibels with respect to 1 volt

dBw - decibels with respect to 1 watt.


Can i use the oscilloscope to measure this different db scales?

how do i do that with a oscilloscope i need measured dbm or dbv or dbw
before with a oscilloscope?

Or i can only use the AC volt meter for doing this?

The post by JimB was very good.

If you use an oscilloscope to measure the voltage you will measure either the peak (Vp) or the peak to peak (Vpp).

If it is a sine wave, then the RMS is Vp/sqrt(2) or Vpp/2/sqrt(2).

From the RMS, you can calculate power P = V^2/R

so dBw = 10 log(P/1)= 10 log(P)

Now convert P to mW.

so dBm = 10 log(P/1)= 10 log(P)

For dBv, the reference is 1 Volt, so dBv = 20 log(V)

Len

 

[walters]

Thanks guys

So i have to measure voltage and then convert and do some formulas
to get dbs

 

[ljcox]

Yes.