Ground and AC Ground

One point of knowledge about circuits is "AC ground." This is an abstract concept. Let's try it with multimeter photos.

Above is the meter lying on the ground, literally. The red probe is measuring the voltage on a nail that is stuck in wet dirt. The voltage is .0037VAC, quite small. Away from the apartment building, this would be about zero. The point is that real dirt is considered to be "ground." The ground hole in AC outlets, below, is wired to a ground stake just outside the building.

Do not try this at home unless you really know what you are doing. (And be sure you have the leads in Common and V-ohms-mA, and set meter on Volts AC). The meter is measuring the voltage on "Neutral." This voltage is higher when a lot of current is flowing through appliances and air conditioning. Since there is a lot of voltage on Neutral, it is not an "AC ground."

Above is inside a homemade microphone amplifier. The red probe (seen) is measuring the filter capacitor in the power supply. This capacitor receives high current pulses, and the AC voltage is .24VAC. This is not considered to be an AC ground even though there is a cap to ground, the normal means of making an AC ground. The current pulses are just too high.

The meter is now moved to the output of the series regulator, and the AC voltage is only .006VAC. This, the +14.7V supply, is a nice AC ground. This is true even though there is a lot of DC voltage on the white banana jack. What makes an AC ground is low AC voltage. Refer to the schematic below.

The schematic above is the positive supply for the two photos above. The filter cap has a lot of ripple voltage and does not qualify to be an AC ground. The output of the regulator has a low AC voltage and does qualify to be an AC ground.

The common-emitter amplifier schematic is ready to build, it has all the bias parts shown, though the resistor values and cap values have to be figured out. There is a reason that the emitter resistor is paralleled with a resistor and cap, and it is to improve the gain, but that is another conversation. The point here is that the cap establishes an AC ground, as marked, which places the two emitter resistors in parallel for gain purposes.

This differential amplifier is designed to operate without a need for AC ground. The schematic below is a better design that does use AC ground. Here is a conversation about the differences.

Both amps have the bases at the same voltage, a basic requirement for diff amps. Above, they are both at ground, and below they are both at the 4V bias.

Above, a -9V battery is needed for the emitter current, and a separate battery is needed for the collector supply. At $3.50 per 9V battery, this adds so much to the cost.

This diff amp, the better design, needs only one battery. The design meets the requirements for transistors, collector more positive than base, and emitter more negative than base. This is done by doing a voltage divider to establish the 4V bias. (The web site has a page about voltage dividers.)

The 4V is made an AC ground by connecting a cap to ground.

The weather radio is battery powered. It needs no connection to ground. Even so, you can say that the negative battery terminal is an AC ground.

When participants in Solder and Circuits design circuits, the use of AC ground is an important concept. This web page provides examples to help participants learn what this is all about.