I'm new to Tina-ti, so thanks in advance for any help you may be able to give me.
I have simulated the drive stage for an amplifier of a real circuit for a unit which checks conductivity of boiler water. The unit uses a constant amplitude drive of a 1kH (400mV p-p) triangle waveform signal into a load, with a feedback loop to ensure that the signal is always constant as the resistance of the load or wiring losses change (from a few ohms to several k).
In the simulation I have used a signal generator to produce a triangle waveform of 68mV p-p at I/P of the amplifier, and this produces a constant O/P of 400mV p-p (as the real unit does).
The simulation works fine & gives me the same results as the real unit. But I need to make some changes to the amplifier, but before I do I would like to try & look at what the phase margin of the loop is.
Here's my issue, if the I/P is 68mV & O/P 400mV, this gives a fixed gain of around 15.92dB and I I double the sig gen amplitude, the O/P doubles accordingly. But the AC Transfer Characteristics tool (see below), shows the gain as never getting much above 0dB.
The first plot below is for a load is for a load of 220Ω & a simulated cable resistance of 100Ω. The second plot is for a load of 22Ω & a cable resistance of 10Ω.
Am I being stupid here & not seeing the wood for the trees here ? So any help would be brilliant.
Hope this is sufficient detail, but I can load the cct if that would help.
From what you mentioned everything seems ok and you should get a gain of ~16dB. Lets try taking a look at the circuit you have to get a better idea of your setup and run simulations on my end. Upload your cct and then we can take it from there. Hopefully we can pin down what's making your gain less than 0dB.
5852.BC3250 TDS Drive Stage.zipThanks Marcos,
Here's the cct file.
Oops, meant to upload a PDF of the cct as well as the Tina.ti Schematic file (which I uploaded twice).
3225.BC3250 TDS Drive Stage.pdf
I ran a few sims on your circuit and I think the reason why your AC doensn't seem to match the transient gain is because of the operating point of the transistors TR2 and TR3.
If you run a DC operating point on the circuit you will notice that these guys are a bit far from being on the verge of the linear region. This creates a big deadzone in your output
and for a small signal (AC) analysis the simulator might be linearizing your circuit around that deadzone where the transistors have very small gain. If you drive your ckt with a small
triangle wave you will notice that the output is smaller than the input. Again, this is my main source of suspicion and what I've gotten so far from tinkering with this puppy. If you feel
like I made a mistake or arrived at a wrong conclusion feel free to repost and we can keep working on the circuit.
Many Thanks Marco,
That was really well spotted; I think you may be onto something there.
As it happens, the O/P transisters I used for the simulation are not the devices that are actually used in the product (because I couldn't find them in the transistor selector) in TINA-ti. They are actually: TR2 (FMMT497) & TR3 (FMMT597); which are two SOT23 devices from DIODES Inc.
Perhaps if I had two transtors in the simulation that were a lot closer to the real ones, may be this would make the simulation work ??
Based on your suggestion, I searched around & found the correct models for TR2 & TR3, as well as bias diode D12.
This made a big improvement, as you can see from the attached AC Analysis & cct
Although I’m still not quite sure why it only shows the expected gain of around 16dB, at a frequency around 280Hz, when the sig gen at the front of the circuit is running and producing its amplified output at 1kHz ?
Many Thanks again for your help in this.
Bill5226.BC3250 TDS Drive Stage (4).zip
I don't have a clear answer at this moment, I will have to dig into this and find a solution at a later time. I apologize for the inconvenience.
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