AB amplifier, am I missing something? - Page 1
Forum section: electronics / beginners

author: TNb
author_rank: Regular Contributor
timestamp: July 08, 2014, 07:20:57 pm
content: Hi! I'm learning A, B, AB amplifiers now and I don't get something.In this video at 35:06 the input and output of AB amplifier is shown.I probably miss something very basic and shameful, but how is that input is 930mV and output is 760mV? Isn't it supposed to be an amplifier?Schematics is here on the second page:http://www.thesignalpath.com/uploads/Class_AB_Amp.pdf

author: homebrew
author_rank: Frequent Contributor
timestamp: July 08, 2014, 07:30:18 pm
content: Quote from: TNb on July 08, 2014, 07:20:57 pmAn amplifier does not necessarily have a voltage gain. This particular one certainly has a current gain (high impedance input) ...

author: TNb
author_rank: Regular Contributor
timestamp: July 08, 2014, 07:36:29 pm
content: that was what I thought... just strange that the author of this video does not show this for the sake of it.But is current-gain amplifiers work similarly concerning speakers, does it matter for the speaker if it is high voltage or high current?You just take V*I and get the output power to the speaker?

author: homebrew
author_rank: Frequent Contributor
timestamp: July 08, 2014, 07:50:46 pm
content: Quote from: TNb on July 08, 2014, 07:36:29 pmYes, in principle, that is correct. In practice however all sorts of power measures are considered for marketing reasons, like PMPO, Sine wave, RMS.Quote from: TNb on July 08, 2014, 07:36:29 pmWell, in general, audio amplifiers have a high current gain. The input is generally high input impedance (some 10k) and very low output impedance (speaker impedance 2-8 ohms). However, given that 2-8 ohms you won't reach the desired output power levels, without also having a voltage gain.

author: cyr
author_rank: Frequent Contributor
timestamp: July 09, 2014, 06:15:42 am
content: Quote from: TNb on July 08, 2014, 07:36:29 pmThe relationship of voltage and current depends on the impedance of the speaker (4 ohm, 8 ohm, whatever).An audio amplifier will typically have multiple different stages, to provide both voltage and current gain. The circuit you linked could be the final output stage.

author: ConKbot
author_rank: Super Contributor
timestamp: July 09, 2014, 11:42:54 am
content: The linked schematic in the first post will work, but its more of an example circuit, or a building block than something thats ready to be used. However, given that there is no 'global' feedback, just the local feedback of the transistors, with correct biasing a simple transistor circuit like that would be stupidly high bandwidth compared to an audio amp.http://sound.westhost.com/project3a.htmis an example of a buildable AB power amplifier. It looks complex but it is actually pretty simple if you break it down into functional blocks.Q1/Q2 is a long tail pair, which functions as a differential amplifier, the bases of the transistors are the inverting and non inverting inputs, just like on an op-amp, and the collector of Q1 is the output. Q3/R7/D1 make up a constant current sink for the LTP, with the LED being used as a voltage reference so the current would be (Vled-Vbe)/R7Q4 provides current gain to supply current for the resistor string and the next set of transistors. Q9/VR1/R16 adjusts the DC operating point by affecting how much the transistors are turned on with a 0V input. The more the output transistors are on at a 0V input, the higher the idle current of the amplifier, but it also generally reduces crossover distortion.Q5/Q7 and Q6/Q8 are the output drivers and output transistors, arranged to the schematic in the first post. Then the output signal goes though R5/R4 which divide the signal back down (C3 blocks DC, so only AC signals get gained up, DC signals have a much lower gain, which helps control DC offset) before feeding it back into the long tail pair to provide global feedback. R15/C7 is the zobel network, which helps stabilized the amp with inductive loads, and C+ and C- are just simple decoupling caps for high frequency noise. This would run off a unregulated transformer/rectifier, and the amplifier power supply rejection ratio (PSRR) is what keeps the output quiet.Even if it doesn't all make sense, hopefully you got some new keywords to look for.

author: pinyoro
author_rank: Contributor
timestamp: July 09, 2014, 01:13:55 pm
content: Maybe he is using *10 setting on his probes?Sent from my RM-825_eu_euro1_234 using Tapatalk

author: TNb
author_rank: Regular Contributor
timestamp: August 02, 2014, 09:50:46 pm
content: Ah, sorry I wasn't replying for a long time, was on holidaysAnyway, just wanted to say thank youSorry for putting this topic up...