Ground and 0v (been asked before, but confused still)

Thread Starter


Joined Mar 26, 2009
It's been asked before and i've been looking at previous threads but the fundamental's of AC in a DC circuit is just giving me grief.

Ground and 0v DC (are not the same but similar?) So take a look at the image i included... now here's what I don't get, you'll see I set it to output a sine wave, I then tried to DC rectify it somewhat by using a single diode and 1 hert, and they i played with some ridiculously values, meanwhile trying to find out what effect an inductor does in series with a large cap.

Now why does this circuit need ground? and in the real world where does Ground come from? i was expecting to be able to use the Sine Wave and make a ground, but 'Computer says no' or rather LTspice does.. all this came about because I was trying to work out how to amplify the signal with just a simple transistor (Obviously, depending on the Frequency and Transistor, it might not be able to switch fast enough)..

So I stuck with a low low radio frequency, around the 300khz range, now this has NOTHING to do with radio, after giving up on that I then took the output from my MP3 player.. still could not get it working, the DC/AC and joining them together is confusing..

So taking an MP3 player output, I have Common which links to both the Left/Right channels, but is Common 'ground'?

Common to 0v?
Left/Right Channel to the base of the Transistor?

All i know is i've had a lot of smoke, the idea was to increase the volume, I know that the wrong transistor is bad, eg switching speed, noise being produced etc...

But surely it's possible to to gain the imput from a single transistor? I don't mind that it crackles or you can barely hear it, I just wanted to get a small gain boost on the out, and I'm not sure which outs need to be connected where, what's the basic theory?

So back to my LTSpice, why is Ground needed? -12 to +12dc rectified can't I use output sinewave to make my own ground? otherwise where is ground in real life? 1.5v battery for example, ground is the negative terminal, where is ground on an AC sinewave output?

I'm basically confused, DC makes sense, integrating AC into DC is the issues im having, DC to power the circuit, DC to feed to the Transistor, Collector/Base/Emitter where's the AC output connect in a DC circuit? All i've managed to do is blow a sound card up... long story and I also managed to blow an op-amp because of wiring.... what am I missing here?


Joined Dec 26, 2010
You really need to start with basics and work slowly forward. Trying to understand everything at once will lead to confusion.

Simulation programs like PSpice calculate potentials (voltages) with respect to a common ground reference. The user is required to specify a point in the circuit to be the ground. This point may represent a ground connection in a real-world circuit, but not necessarily - it may be an arbitrary choice. It is not optional however, you must always connect a ground somewhere. Generally, all parts of the circuit to be simulated must have a DC path to this ground reference.

It is indeed possible to build a simple amplifier circuit using just one transistor, but you cannot just connect up devices randomly and expect them to function. Transistors require to have correct working conditions so that they can operate effectively and safely. You might start by looking up the topics described on this web page, and following sections.

I note that you seem to have a particular problem with AC coupling, which is commonly used for amplifiers which do not need to amplify a DC signal. There is no great mystery here: the transistor (or other amplifying device) is supplied with power and bias so that is capable of amplifying. Coupling components (often capacitors, sometimes inductors or transformers) are used to separate the AC signals from the DC.

In some cases connecting the AC signals directly to the amplifying device would disrupt its DC conditions: many AC sources and loads can also pass DC. In other cases the DC voltages on the amplifying device would have bad effects on whatever it is connected to. This can make it necessary to separate the AC signals from the DC.

Understanding transistor amplification and biasing may take some time and effort, but in the long term it will be very useful. Until you have mastered these ideas, it may be helpful for you to try building or simulating circuits which other people have already designed for you.


Joined Mar 24, 2008
The concept of ground has built up over the last century.

Originally it was earth, literally a metal spike driven into the ground.

However, as electronics has evolved it took on another meaning. It is the 0V reference, the point all other voltages are measured from.

The two have a passing resemblance to each other, though one evolved into the other.


Joined Dec 26, 2010
Even if a circuit operates in a spacecraft, if we are going to make calculations it will be useful to have a 0V reference.

For anything more than a trivial circuit (like a battery and a bulb) there may also be a functional need for a common point, such as a supply rail to which many currents are returned. For circuits handling signals, whether analog or digital, it is usual to have a common reference. This is sometimes linked to the potential of surrounding metalwork, but by no means necessarily so.

There is a rather different use of actual ground connections in mains power systems. This is concerned with safety, as a method attempting to minimise the danger arising from faults. Signal "grounds" are often kept separate from the mains ground, or if it is necessary to join them for safety the connection is made at one point in the circuit only. Multiple connections between signal and power grounds can have harmful effects.
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Joined Mar 24, 2008
Still, it is about a common measuring point. That is the unifying thread. The subject isn't always clear cut and concise. In some cases digital and analog circuits sitting side by side have two different ground points, both separate, to prevent interference with each other.

To the OP, ground is not complex in simple circuitry. It is merely the point where the negative lead of the DVM goes to measure all other voltages, both AC and DC.

As I said before, the concept evolved into what it is today. Dealing with the older concept, most appliances that have a metal box (such as washing machines) the outside box or chassis is grounded. If you think about it you can see how the transition was made to modern electronics.


Joined Dec 26, 2010
Here is the result of a simulation of a very simple transistor amplifier circuit. Note that all voltages are referenced to ground.

The input voltage Vin and the output voltage Vout have no DC content. The base voltage VB and the collector voltage VC have mean DC voltages appropriate to the transistor being biased to give about 1mA collector current.


Joined Jan 11, 2009
there is no voltage at a point. although we often talk as if there is. voltage is really a measurement in volts of a potential difference. for a difference you need 2 points. if you take some point ( any point in the circuit ) and call it 0 then you can measure the voltage at any other point, and talk about the voltage at a point. sometimes we drive a stake in the ground and call it 0 sometimes we take the negative side of the battery.