12
\$\begingroup\$

Is there any standard in industry for using uppercase or lowercase for V and I in circuit diagrams? I ask this because my book seems to switch back and forth between the two without any rhyme or reason and I can't figure out any pattern for why it will choose one way over the other. It also switches back and forth for subscripts.

\$\endgroup\$
  • \$\begingroup\$ Can you show an example? \$\endgroup\$ – Roger Rowland Jun 29 '15 at 5:49
  • \$\begingroup\$ I was just wondering if there's a standard to go by, I don't have good lighting at the moment to take pictures to compare. If no one answers by morning I will though. \$\endgroup\$ – Austin Jun 29 '15 at 6:06
13
\$\begingroup\$

I agree with you that it is important to know the meaning of the different ways for using such symbols. And the same applies also to the voltage-to-current ratios (resitances, impedances). For my opinion the standard is (or should be) as follows:

  • Uppercase (V,I) for DC and rms values
  • Uppercase for ohmic resistors R=V/I
  • Lowercase (v,i) for signals as a function of time: v(t), i(t)
  • Lowercase (v,i) for (small) differential signals available for a certain DC bias point only.
  • Lowercase (r) for differential (dynamic) small-signal resistances r=v/i.

As a negative example, in small-signal equivalent circuits, sometimes the inverse transconductance gm of a BJT is used as Re=1/gm. This is very confusing because this does not represent any (ohmic) resistor and, more than that, can be mixed-up with an external emitter resistor RE.

EDIT/UPDATE: Regarding impedances:

For reactive elements (L, C) the voltage-to-current ratios are called "impedance". Because this applies to rms values of sinusoidal signals only the symbol for impedances also is written in uppercase letters Z=V/I.

\$\endgroup\$
  • \$\begingroup\$ +1 @LvW, I'd like to request OP to take this up as an answer. The answer provided by me is not so specific and rather a link-only kind, while this one is right to the point. \$\endgroup\$ – WedaPashi Jun 29 '15 at 6:34
  • 1
    \$\begingroup\$ Didn't see it at first, but this is exactly the info I needed. Thanks! \$\endgroup\$ – Austin Jun 29 '15 at 6:35
5
\$\begingroup\$

I'll add a couple of points to the good answers you've already received:

quantities obtained applying various kind of transforms to the time domain signal should be uppercase, specifically:

  • Phasors, i.e. complex representation of sinusoidal signals.

  • Frequency-domain signals, i.e. Fourier transforms of time-domain signals. For example the frequency response of a system and related input and output signals or the like:

    • \$Y(f) = H(f) \cdot X(f) \$
    • \$V_o(\omega) = G(\omega) \cdot V_i(\omega) \$
  • s-domain signals, i.e. Laplace transforms of time-domain signals. For example the transfer function of a system and related input and output signals or the like:

    • \$Y(s) = H(s) \cdot X(s) \$
    • \$V_o(s) = G(s) \cdot V_i(s) \$
  • z-domain signals, i.e. Z-transformed signals corresponding to discrete-time signals (e.g. as used in digital signal processing). For example the transfer function of a digital system and related input and output signals or the like:

    • \$Y(z) = H(z) \cdot X(z) \$
\$\endgroup\$
3
\$\begingroup\$

I can't find a link to a book that I had referred that talked of such notation conventions. But the closest that came to that standard books was this wikipedia page.

It says,

Large-signal DC quantities are denoted by uppercase letters with uppercase subscripts. For example, the DC input bias voltage of a transistor would be denoted VIN.

Small-signal quantities are denoted using lowercase letters with lowercase subscripts. For example, the input signal of a transistor would be denoted as vin.

Total quantities, combining both small-signal and large-signal quantities, are denoted using lower case letters and uppercase subscripts. For example, the total input voltage to the aforementioned transistor would be : vIN(t)=VIN+vin (t).

And,

A large signal is a DC signal (or an AC signal at a point in time) that is one or more orders of magnitude larger than the small signal and is used to analyse a circuit containing non-linear components and calculate an operating point (bias) of these components.
A small signal is an AC signal superimposed on a circuit containing a large signal.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.