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tca 6th July 2012 02:53 PM

Direct-coupled amplifier circuit
I was browsing the google patents site and found this Direct-coupled amplifier circuit. The circuit is this

Is there other way of getting a direct coupled output from a class A amplifier with out dynamical changing the bias of the amplifier? Comments on the schematic?


Osvaldo de Banfield 6th July 2012 03:20 PM

I canīt understand what are trying to do exactly, but the mod I may suggest is to replace the bias circuitry by an opamp wired as inverting low pass filter, tie the + input to a reference supply for the amp, and the output by means of a resistor to the base of first transistor. But it is normally unnecessary such a circuit for a simple preamp.

bear 6th July 2012 05:14 PM

Good to read the text of the patent.

Looks like the intended application is for high speed amps, video amplifiers are mentioned in passing. The LPF feedback is essentially a DC servo. The input is said to require a blocking capacitor.

The main feature is simplicity and the idea that the feedback loop automatically adjusts the operating point to 1/2 Vcc.

Beyond that, there must be some application that is left unstated where this sort of arrangement is beneficial in practice.

I did not read all the way through to see if there are other words that hint at what that may be. Also it might be instructive to search for other patents by the same team during that time period to see what they were going for.

This may have application to DC servo methods.

The usual means of getting DC coupling on the output stage of a class A or AB or B amplifier is to control the DC offset of the amplifier. That can be by dynamic means (usually the "DC servo" = LPF fed back to the input or other suitable point) or static means (which is just a really stable non-drifting circuit). Both exist. Pretty common.

Same thing regardless of P-P or SE.


tca 6th July 2012 10:44 PM

Hi bear, thanks for your reply. Can you give me some references on DC servo methods, book or on-line, would be great. Thanks.

bear 7th July 2012 01:47 AM

search this site... that would be a start.

the idea is simple enough - if you sample the output of an amplifier, it will have DC + AC signal. we are only interested in the DC. IF you can separate the two, then you can take the DC, which can have three states: minus, zero, and plus, and feed that signal in proper proportion back to the input of the amp, or other suitable point.

That signal that is the offset voltage (DC) is then usually inverted, so that it is applied to the input, causing a shift in the output stage in reverse of the polarity of the offset - nulling it. It tries to find "zero" as a result.

The way this is done is to form a low pass filter at <1Hz. which effectively eliminates all the AC signal from the feedback path.

This is a separate feedback from the main feedback, usually.

There are a variety of means to make this happen... you can likely find a bunch of them here. They are also mentioned elsewhere on line. Just search for "DC servo" and "amplifier" should come up with a bunch.


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