effects of mixed AC and D.C. Signals on same conductor

Thread Starter

pathos

Joined May 24, 2013
12
Hi folks,

I have been working/ building some oscillators recently in efforts to learn more about signals in general. I am wondering about the phenomena of mixed signals and how they influence each other.

I've just read the article on this site in mixed signals and it was quite informative on when such signals are undesirable, but I'm more interested now in those that are desirable. For example, I am looking at an electret mic input to an audio transistor base, an AC signal. The base I'd also biased by a voltage divider circuit at the same point. Are both signals on the same wire at the same time and while serving two totally different purposes, do they affect each other? I could see the D.C. In general increasing the amplitude of the AC waveform, but is there more to consider?

As always, thanks for any insights.

Dave,
Fort Collins CO
 

Reloadron

Joined Jan 15, 2015
7,501
I can give you an example of where we may wish to "offset" an AC signal on a DC level.
Current Transformer Amp2.png

This is a simulation of using a CT (Current Transformer) to measure an AC current. I1 is a current source of 5 Amps, a typical CT output. The 5 Amps is passed through a R1 0.005 Ohm resistor acting as a burden resistor or shunt resistor. This provides a 0 to about 25 mV signal swinging from -25 mV to 25 mV above and below a zero volt reference. We offset this signal in the operational amplifier circuit. R3 and R4 form a Voltage Divider of the 5.0 Volts (Supply Voltage). so 2.5 Volts is applied to the Non Inverting (+) input of our operational amplifier while out signal is applied to the Inverting (-) input. The end result is the output of the operational amplifier is an amplified and inverted original signal on a 2.5 VDC offset.

There are hundreds of examples where signals are mixed and or offset and using a single conductor. AM (Amplitude Modulation) would be an example of mixed AC signals.

Ron
 

dl324

Joined Mar 30, 2015
16,839
For example, I am looking at an electret mic input to an audio transistor base, an AC signal. The base I'd also biased by a voltage divider circuit at the same point. Are both signals on the same wire at the same time and while serving two totally different purposes, do they affect each other? I could see the D.C. In general increasing the amplitude of the AC waveform, but is there more to consider?
When talking about circuits, it's usually helpful to post a schematic.

For the circuit you described, you probably forgot to mention that the mic was AC coupled to the voltage divider; so DC from the input or output would be blocked.

Since amplifiers require DC bias to function, the situation you're concerned about occurs frequently.
 

wayneh

Joined Sep 9, 2010
17,496
Voltage is always measured and defined relative to a reference. AC voltage is relative to its center zero. It doesn't care if you call that center zero volts DC or a million volts. It's exactly the same AC signal. I'm saying the AC and the DC are largely independent.

Semiconductors, such as a diode junction or the base of a transistor are examples where the DC voltage (base relative to emitter) can affect what happens to the AC voltage and so they're not independent.
 

crutschow

Joined Mar 14, 2008
34,280
In summary, there is no significant interaction between AC and DC signals inside a wire.
You can treat them as independent entities.
 

#12

Joined Nov 30, 2010
18,224
There is a thing called a DC coupled amplifier where the DC level of the first stage is intentionally used as the DC operating point for the next stage. Sometimes you use both components of the signal, and sometimes you don't. If the DC level in a wire is not what you want for the next stage, you use a capacitor to block the DC, pass the AC, and set up the DC bias level you want with a couple of resistors on the input of the second stage of the amplifier as you described in your first post.

Think of frequency as a continuum from zero to gigahertz. DC is a frequency of zero cycles per second. It is easy to block the DC because of the hard zero in the equation for the frequency dependent impedance of a capacitor. As for what the two frequencies do to each other in a wire? Usually, the answer is, "nothing". You have a wire with more than one frequency. You can select what parts to use with low pass filters, high pass filters, bandpass filters, and filters work all the way down to zero Hertz. The frequencies in a wire are not fighting each other. They just exist and you design your circuit to use or discard which ever frequencies you want.
 
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