Perform a transient analysis on it. It may take awhile to start oscillating. Try 200mS with a max timestep of 20uS. Start the supply at 0v, and skip the starting operating point. L2 seems to be VERY large. You might help start the oscillation by using an initial condition statement: .ic i(L2)=1uA Your OUT node is indeed floating. Connect a high-value resistor (say, 1MEG) from OUT to Ground. [eta] I ran a simulation of your circuit in LTSpice; it oscillated at ~17.4kHz. It starts just fine without the startup or skip initial operating point options, and the .ic wasn't necessary either.
what about dc analysis then? they say that the only analysis i can do is dc and transient but i need ac too.. x.x im not familiar with oscillators.. x.x help? im at lostx.x
For AC analysis, you typically place an AC source at the input, often set to 1V amplitude, so that the AC Analysis can display the ratio of the AC output to the AC input over some range of frequency. It displays the "transfer function". But your circuit does not have an input.
ive chosen another ckt for my project in pspice, cos we are required to have 3 analysis x.x ive chosen an instrumentation amplifier.. x.x im at loss now, x.x
can anyone help me? i need at least 5 objectives, the analysis works but i dont know what to sweep/ where to put the markers.. x.x
According to "A Designer's Guide to Instrumentation Amplifiers", by Charles Kitchin and Lew Counts: "Common-mode rejection, the property of canceling out any signals that are common (the same potential on both inputs), while amplifying any signals that are differential (a potential difference between the inputs), is the most important function an instrumentation amplifier provides. Both dc and ac common-mode rejection are important in-amp specifications. Any errors due to a dc common-mode voltage (i.e., a dc voltage present at both inputs) will be reduced 80 dB to 120 dB by any decent quality modern in-amp.". Maybe you could compare your instrumentation amplifier's performance/specs to those of a single-opamp amplifier, by simulating both. For more about in-amps, you can find that paper on line, probably at analog.com. They also have lots of other material about instrumentation amplifiers. That paper also says this: "In order to be effective, an in-amp needs to be able to amplify microvolt-level signals while simultaneously rejecting volts of common-mode at its inputs. It is particularly important that the in-amp is able to reject common-mode signals over the bandwidth of interest." So maybe your first objective should be to characterize the gain bandwidth of the instrumentation amplifier, i.e. the difference between the frequencies of the upper and lower -3dB gain points, which you can get from AC Analysis. You would want to at least find the two -3dB frequencies, the gain, and the difference between the two frequencies (the bandwidth). Then maybe you could characterize how well the AC and DC Common-Mode Rejection work, over that bandwidth. More about that, from the same paper: "Common-mode gain (ACM) is related to common mode rejection and is the ratio of change in output voltage to a change in common-mode input voltage. This is the net gain (or attenuation) from input to output for voltages common to both inputs. For example, an in-amp with a common-mode gain of 1/1,000 and a 10-volt common-mode voltage at its inputs will exhibit a 10 mV output change. The differential or “normal mode” gain (AD) is the gain between input and output for voltages applied differentially (or across) the two inputs. The common-mode rejection ratio (CMRR) is simply the ratio of the differential gain, AD, to the common-mode gain (ACM). Note that in an ideal inamp, CMRR will increase in proportion to gain." I would think that you should be able to apply both common mode and differential mode signals and plot the output for each (with AC Analysis, I assume), to show the common mode gain and the differential mode gain, versus frequency, over the entire bandwidth. And if there is a way to divide one by the other and plot the actual CMRR (i.e. diff-mode gain plot divided by common-mode gain plot, vs freq), so much the better! (But you might have to make two identical circuits in the same schematic, to be able to have both at the same time, so you could divide one by the other and plot the result (for both AC and DC cases separately, of course). [Or at least that's the only easy way I can think of, to do that.]) P.S. If you are not absolutely required to use a 741 opamp, use a more modern one! The 741 is terrible. Cheers, Tom
So maybe your first objective should be to characterize the gain bandwidth of the instrumentation amplifier, i.e. the difference between the frequencies of the upper and lower -3dB gain points, which you can get from AC Analysis. -- how can i do this in ac sweep then? x.x sorry, im just not familiar with pspice.. x.x
You will have to LEARN all about pspice. You can ask for help. But no one can do most of the work FOR you, even if they wanted to try. Have you tried reading the Help files, or the manual, or a book about it, or some of the many on-line tutorials? It is not very difficult, once you get started. But it WILL require your TIME. You MUST be willing to invest almost ALL of your time in your studies. That is the only good way to do well and succeed. Good luck! Tom