converting Volt into dB - Page 1
Forum section: electronics / beginners

author: Mepakos
author_rank: Contributor
timestamp: August 06, 2019, 09:14:46 pm
content: How can i convert -20V into dB?

author: magic
author_rank: Super Contributor
timestamp: August 06, 2019, 09:34:04 pm
content: You can't, logarithm is only defined on positive numbers. Decibel scale is usually used to refer to signal power and power is always positive too.It's you the Decibel Guy again, I see. You would get more sensible responses if you explained what you are trying to calculate and why instead of asking one-liner questions that don't even have a valid answer.

author: Mepakos
author_rank: Contributor
timestamp: August 06, 2019, 09:46:58 pm
content: The question is what is the voltage gain of the transistor in dB? its -20V! I need to convert that to dB

author: Kalin
author_rank: Regular Contributor
timestamp: August 06, 2019, 09:50:27 pm
content: No it is not -20V. People are not going to do your homework for you. If you want to know how something works or where you went wrong in a calculation many people will help you, but noone is going to just give you answers. Gain is a ratio of input to output and doesn't have a unit.Sent from my SM-G965W using Tapatalk

author: Mepakos
author_rank: Contributor
timestamp: August 06, 2019, 09:55:30 pm
content: I dont ask for answers just for good advice on how to solve the task.Instead of saying u are not doing my homework which i dont want, u can tell me why -20V is wrong. Ua=20Vss and Uce=Ua. UCE is the voltage gain of the transistor. So why its wrong.

author: Kalin
author_rank: Regular Contributor
timestamp: August 06, 2019, 10:03:02 pm
content: https://en.m.wikipedia.org/wiki/Gain_(electronics)Read this and report back what you think your gain is and how you think you would convert that to dBSent from my SM-G965W using Tapatalk

author: Mepakos
author_rank: Contributor
timestamp: August 06, 2019, 10:18:38 pm
content: 10log(20/1)=13,01dB.

author: Bassman59
author_rank: Super Contributor
timestamp: August 06, 2019, 11:44:45 pm
content: Quote from: Mepakos on August 06, 2019, 09:14:46 pmWhat is the reference to which the -20 V is compared?

author: chrisl
author_rank: Regular Contributor
timestamp: August 06, 2019, 11:54:54 pm
content: Quote from: Mepakos on August 06, 2019, 10:18:38 pmThat is a power gain in dB. What you are looking for is a voltage gain in dB.

author: Audioguru again
author_rank: Regular Contributor
timestamp: August 07, 2019, 12:23:56 am
content: He is wrongly subtracting the gain number because the transistor inverts the signal. The voltage gain is still 20 times whether it is inverted or not.A voltage gain of 2 times is +6dB.A voltage gain of 10 times is 20dB.Then isn't a voltage gain of 20 times 6dB added to 20dB= +26dB?

author: bob91343
author_rank: Super Contributor
timestamp: August 07, 2019, 12:26:06 am
content: Strictly speaking, dB has to be power gain. Output power divided by input power gives the power gain ratio. Take the logarithm of that (base 10) and multiply by ten to get the result in dB.However, the situation is more complex, as the definition has been perverted. One way it's been perverted is to ignore the impedance, thus destroying the meaning. Another way is to assume a reference that, for some reason, isn't stated. Example might be sound levels, which are usually expressed in dB but no reference given. There is an assumption but it's seldom stated.Standard references are 1 milliwatt, 6 milliwatt, and I forget the sound pressure level, something like 0.0002 dynes per sq cm but don't quote me. Sometimes 1 Watt is the reference.For radio communications, the reference is 1 milliwatt. It's indicated by, rather than dB, dBm. In a 50 Ohm system, -73 dBm is 50 microvolts. (Also referred to as signal strength S9)So try to keep the air clear and be precise in what you mean.

author: radiolistener
author_rank: Super Contributor
timestamp: August 07, 2019, 01:29:04 am
content: Quote from: Mepakos on August 06, 2019, 09:55:30 pmbecause input voltage is Vin = 1 mV AC and output voltage is Vout = 20 mV AC. So, voltage gain is:Vgain = Vout / Vin = 20 / 1 =20 timesQuote from: Mepakos on August 06, 2019, 09:46:58 pmThere is 1 mV AC on the input and 20 mV AC on the output. So we have voltage gain:Vgain = Vout/Vin = 20/1 = 20 timesYou can convert it into dB in the following way:Vgain_dB = 20 * log10( voltage_gain ) = 20 * log10( 20 ) = 26 dBThis equation 20*log10(x) is actual for voltage and current gain. But when you needs dB for power gain, then you're needs to replace 20 with 10:Pgain_dB = 10 * log10( power_gain )For example, if you have 5W on the input and 30W on the output:Pgain = Pout/Pin = 30/5 = 6 timesPgain_dB = 10 * log10( 6 ) = 7.7815 dB

author: magic
author_rank: Super Contributor
timestamp: August 07, 2019, 07:33:15 am
content: Quote from: bob91343 on August 07, 2019, 12:26:06 amAll true.And one way it was perverted, and the way OP probably is interested in, is in using dB to refer tovoltage gain, without taking into account load resistance and therefore actual power output of the amplifier.For this purpose, you simply assume that increasing RMS voltage of the signal 20 times increases power 20·20=400 times. So that's 52dB gain. Everybody reports voltage gain that way and virtually no one will say that the gain of your circuit is 26dB.I think this perversion originates from pro audio, where load impedance used to be assumed 600Ω for some reason (could it be the typical impedance of input transformers in old gear which employed them? no idea). If load impedance is constant and if the amplifier is able to drive it then of course 10x voltage gain produces 100x power gain so this "perverted" rule makes sense.Also, note that power is always positive. So it doesn't matter if the amplifier is inverting or not. In particular, -52dB gain means that output is 1/400 of the input.Quote from: bob91343 on August 07, 2019, 12:26:06 amIndeed. If you want to accurately reportpower outputof this amplifier, you multiply output RMS voltage times output RMS current into whatever load will be connected. That's your power which you express in dB referred to watt (dBW) or milliwatt (dBm) or whatever. For example, 20V RMS into 50Ω is 8W so probably about 8dBW=38dBm or so.If you want to reportpower gain, you divide output power as above by input power going into the amp and get a unitless (just dB) gain ratio.

author: NaxFM
author_rank: Regular Contributor
timestamp: August 07, 2019, 02:44:57 pm
content: You can't have a negative gain, the gain is always positive, less than one if it's an attenuation, more if it's a gain.Also, the gain in this case is a unitless number, because it's a ratio between output and input voltage, so the unit cancels out.Doing the ratio, the result is negative, in this case for the gain you just take the absolute value and say that the output is invertedInviato dal mio Moto G (5S) Plus utilizzando Tapatalk

author: bob91343
author_rank: Super Contributor
timestamp: August 07, 2019, 04:31:09 pm
content: The broadcasters' use of 600 Ohm stems from telephone engineering. The telephone systems are traditionally 600 Ohms, most likely derived from practical considerations of building the transmission lines. They were balanced lines.Most broadcasters had some link to the telephone system, primarily for long distance relay of programs. So they had to match the line to avoid reflections (echo).So the terms dbW and dBm and so on referred to the base, or 0 figure. (Note that the Bel was named after Alexander Graham Bell, and so the B needs to be capitalized. The leading d means one-tenth and so is lower case. The m suffix means milliwatt. The W suffix means Watt, after James Watt. Capitalization is important and often neglected.)

author: soldar
author_rank: Super Contributor
timestamp: August 07, 2019, 06:28:15 pm
content: Quote from: NaxFM on August 07, 2019, 02:44:57 pmI don't think that's right.https://en.wikipedia.org/wiki/Decibelshows negative numbers. 0 dB is gain of unity. Negative gains get negative numbers in dB. WIFI ranges in the -60 dB range. Audio too.

author: NaxFM
author_rank: Regular Contributor
timestamp: August 07, 2019, 07:25:52 pm
content: Quote from: soldar on August 07, 2019, 06:28:15 pmWell, yes, speaking in terms of deciBels you can have negative gain. I didn't say that, but I was talking about gain expressed as a pure number.If the ratio is less than one, then the gain in deciBels is negativeInviato dal mio Moto G (5S) Plus utilizzando Tapatalk

author: radiolistener
author_rank: Super Contributor
timestamp: August 09, 2019, 03:31:50 am
content: Quote from: soldar on August 07, 2019, 06:28:15 pmNegative gain in dB is not a gain, this is attenuation

author: soldar
author_rank: Super Contributor
timestamp: August 09, 2019, 06:39:49 am
content: Quote from: radiolistener on August 09, 2019, 03:31:50 amAttenuation is the same as "negative gain" and both are correct. Just like a negative profit is a loss, negative growth is shrinking, etc.A program designed to calculate gain can output the result as "Gain: -20 dB" and it is not incorrect.

author: mikerj
author_rank: Super Contributor
timestamp: August 09, 2019, 10:38:10 am
content: Quote from: soldar on August 09, 2019, 06:39:49 amExactly, the sign changes depending on whether you refer to a value as a gain or an attenuation.

author: mikerj
author_rank: Super Contributor
timestamp: August 09, 2019, 10:42:47 am
content: Quote from: bob91343 on August 07, 2019, 12:26:06 amI don't understand this, dB is simply a log scale, it can be used with many different units. Are you saying that using it for e.g. electric field strength or bandwidth is wrong?

author: radiolistener
author_rank: Super Contributor
timestamp: August 09, 2019, 11:41:31 am
content: you can use it for different units. For example there is popular unit dBuV (decibel micro-Volt)

author: Audioguru again
author_rank: Regular Contributor
timestamp: August 09, 2019, 02:31:29 pm
content: The inverted output of the transistor has no load so its voltage gain is 20 times which is +26dB. It has some output impedance so its output level and voltage gain will drop when it has a load. Then you can calculate its power gain if you want.edit: Because it inverts, its outputphaseis negative, not its gain.

author: pepelevamp
author_rank: Regular Contributor
timestamp: August 09, 2019, 03:44:46 pm
content: ya know i figure you don't need to be talking about power or what not to use decibels. its simply a way to talk about scales easily. but the power db versus voltage db - that can be confusing.

author: bob91343
author_rank: Super Contributor
timestamp: August 09, 2019, 04:38:26 pm
content: I reiterate that decibels are a measure of power ratio. Any other use is perversion.Having said that, it's popular culture to use decibels for other purposes, although it's clearly wrong.As for using it for field strength, etc., it's still a valid power ratio.We don't say that a 2 Ohm resistor is 6 dB higher than a 1 Ohm resistor. Decibels are properly used for power ratio.Saying an amplifier has such and such decibel gain is improper. But common practice.The bottom line is that you can be precise or not but if your audience understands your meaning you can get away with however you say it.

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 09, 2019, 05:01:41 pm
content: Indeed, one can use decibels for many purposes. Many prepostperous.In my previous job, since it was a public company, the CEO's compensation was available.As a prank, we started calculating how many decibel dollars (dB$) our pay compared to the CEO's compensation.Because it was purchasing POWER, it was calculated as: 10log(my$/$CEO$)Of course, the results were always negative

author: soldar
author_rank: Super Contributor
timestamp: August 09, 2019, 10:27:56 pm
content: Quote from: bob91343 on August 09, 2019, 04:38:26 pmCan we have a cite for that? Because Wikipedia disagrees and citesISO 80000-3:2006.Quote

author: Audioguru again
author_rank: Regular Contributor
timestamp: August 10, 2019, 02:01:47 am
content: The voltage gain of opamps is measured in decibels.The voltage gain of audio power amplifiers is measured in decibels.The voltage cutoff frequency of a filter is measured at -3dB.The voltage reduction of a logarithmic volume control is measured in decibels.

author: soldar
author_rank: Super Contributor
timestamp: August 10, 2019, 11:34:51 am
content: Another cite:QuoteAnother:http://www.sengpielaudio.com/calculatorVoltagePower.htm

author: mikerj
author_rank: Super Contributor
timestamp: August 10, 2019, 06:37:22 pm
content: Quote from: bob91343 on August 09, 2019, 04:38:26 pmYou can reiterate it as many times as you want, but unless you back it up with some actual evidence then I'm just going to ignore it.

author: magic
author_rank: Super Contributor
timestamp: August 10, 2019, 07:21:17 pm
content: QuoteWorth noting that when it refers to voltage gain, it's still reported as the equivalent power gain under assumption of equal load impedance. That is, a 10x voltage gain is reported as 20dB because it corresponds to 100x power gain, not as 10dB because it's a 10x gain.Just Wikipedia being a misleading pile fo turd as usual

author: soldar
author_rank: Super Contributor
timestamp: August 10, 2019, 07:56:09 pm
content: Quote from: mikerj on August 10, 2019, 06:37:22 pmYou pervert, you!