"Transfer Function" explained?

[Netlist]

My sim (and not only mine) can plot transfer functions but I have no clue what exactly is a transfer function or how to interpret the results of the calculation.
The picture shows a diff-amp with calculated transfer function in red.
Can someone enlighten me (and probably many of us)?
When setting the parameter to Output Expression=V(OUTB) the result is -604.23.

Hugo - Calling the EE's...

[biff584]

A transfer function refers to the ratio of the output signal to the input signal and can reflect many things- it can be a ratio of output voltage to input voltage such as the gain on an amp, it can be the ratio of output impedance to input impedance on a filter- basically a transfer function tells you what output you can expect from a given input signal. For example, if you have a transfer function T and an input signal S then the output you would expect from that circuit would be (S x T) -- the product of the transfer function and the input signal.

Brian.

[Netlist]

Thanks, Brian
But what is that transfer function called then? From what I understand is it gain in an amplifier.
Where is it good for? Do we have to assume that more is better or does it have to match something? What kind of value are we aiming for in amplifier design? For example, we want high input impedance and low output impedance. Do we also want a particular value for the transfer function?
In the picture above we got 604.23. Looking at the input signal, I can change that from 0.001V to 1 V, the transfer function stays the same. 0.001 X 604.23 would be 0.6 (db?) and 1 X 604.23 gives 604.23?? (db?)
Looking at Output Expression=V(OUTB) = -604.23 we would get negative db's?
A lot of questions reveal that I still don't get it.

/Hugo - will understand it in the end... with a lot of your patience

[Jocko Homo]

The transfer function also takes into account the frequency dependent parts of the equation, so it is a tad more elaborate than just a simple gain relationship. But for most audio work, gain is sufficient.

If you were designing an active filter, then you would use "transfer function".

A negative number indicates that it is inverted. A number less than one would indicate that it has loss.

Jocko

[PGW]

The transfer function relates the output signal to the input signal.
It is a linear relationship, and in EE-speak is a frequency-domain concept; by wiich I mean it relates only the fundamental frequency components of the output and input signals, and ignores any harmonic (non-linear) contributions.

In other words, it relates the output voltage to the input voltage, usually as a frequency dependent function. To paraphrase Brian's answer, if the input is Vin, at some frequency f1, and the output is Vout at the same frequency, then the transfer function, H, at frequency f1 is

H(f1) = Vout(f1)/Vin(f1)

where I have made each parameter a function of frequency to emphasize the frequency depandence.

A graphical expression of the transfer function is the "frequency response" graph (Bode plots), which often come as magnitude in dB versus frequency, and phase versus frequency.

In the picture above we got 604.23. Looking at the input signal, I can change that from 0.001V to 1 V, the transfer function stays the same. 0.001 X 604.23 would be 0.6 (db?) and 1 X 604.23 gives 604.23?? (db?)

The transfer function stays the same because it is a property of the circuit, not the (input) signal.

Your simulator looks like it is giving you a transfer function number at your test frequency that is a magnitude value. And as Jocko says, the negative number here means a phase inversion between input and output. So you have a gain of 604, which is

20*log(604) = 55.6 dB

(since you are comparing voltages)

with a phase of 180 degrees between input and output.

If you put 1 mV in to your amplifier, you'll get 604 mV out;
if you put 1V into your amplifier, the transfer function will tell you that with the linear gain being 604, you'll get 604V out, but you probably know already that in reality the supply rails will clip your output.

-- John

[Netlist]

Thanks guys,
This is starting to make sense to me now.
Now I understand Jocko, saying that a transfer function less then 1 is no good.
I'll do some more experiments but I understand the basics now and that was the main reason for the question.

/Hugo