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21st November 2008, 05:03 AM  #131 
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Join Date: May 2008
Location: Smoky Mountains , Tennessee

Thank you ,andy.. I thought you had to comment at a
command line or such.. but spice lets you comment directly on GUI, then run the whole thing. I have 2 curves , I assume 1 is "a" and 1 is "b". (gain and phase) also , I would do the same to a circuit I modeled..?? Is there a list of other commands beside gain and phase?? (Is it dot commands??) Now I get it, the commands are computed against the models in the library..(components) in a nice graphical form... OS 
21st November 2008, 05:09 AM  #132  
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Quote:
That's what I did to plot the loop gain in post 111. The best way to quickly inspect and experiment with the relationship between LTP gm / OLG, CLG and Cdom. Cheers, Glen 

21st November 2008, 05:13 AM  #133 
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I "blindman" sees clearly, glen.. I did have a hard time seeing the
degen. resistor values to compare your outputs..but your explaination sufficed.. 
21st November 2008, 05:23 AM  #134  
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Quote:
If you are referring to post 111 the degen resistors were 150 ohms all round. I know with the resolution limit the values are hard (or impossible) to read, but I didn't think it mattered as the important part of that pic (WRT to point I was making in that post) was the loop gain plot. Cheers, Glen 

21st November 2008, 05:32 AM  #135 
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Location: Smoky Mountains , Tennessee

Its nice to see it rather someone just telling you a CM gives you 10db more gain. Is it the difference in attenuation at HF that
you are pointing out.. Can spice also show you phase vs. frequency with other values of Cdom??? Sorry already figured that one out...it can show phases anywhere in the circuit simultaniously if run w/ the same simulation command. 
21st November 2008, 05:39 AM  #136  
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Quote:
As stated I was pointing out the fact that the decreased open loop gain by ditching the current mirror in favor of a pair of load resistors changes the open loop bandwidth, not the unity loop gain frequency (with the the value of Cdom unchanged). Also, if the VAS if buffered with EF's the CM gives much more than an extra 10dB gain (and lower open loop bandwidth), but the Cdom rule outlined in the paragraph above still applies. BTW, you should forget about the collectorbase version of miller compensation with this type of topology all together (it’s not ideally suited) and do what Bob Cordell did in his MOSFET amp. However this is a little more complex to examine in LTspice as you then have 2 loops to inspect. However the feedback cap is really easy to calculate for fixing the desired unity loop gain frequency. I'm busy with other things now so you’ll have to nag Andy C for an explanation on this one Cheers, Glen 

21st November 2008, 05:59 AM  #137 
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You opened up a 'can of worms here' for me..
,spice can show Ic and all other circuit variables Vs. freq. I have only used it to simulate statically because I didn't how to edit a sim command.One can see not only IF a circuit works but HOW it works under a more sophisticated simulation. No wonder you guys are "hooked" on it... 
22nd November 2008, 02:48 AM  #138 
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Location: Smoky Mountains , Tennessee

Well ,I have it..sim city.
It amplified, all currents set (8mA VAS's/ 2ma LTP) balances perfectly .01Ma either side of each.. As far as gain/phase , I,m not sure I did it right, as this amp I modeled in it's entirety, not sure it's only OLG?? I,m only starting but have all the favorite trannies in my model library (had to figure that one out) and can really "pump out the circuits" now. thanks andy and GK,i,m really hooked. OS 
22nd November 2008, 03:28 AM  #139 
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Looks like you're making good progress there ostripper.
If you change that plot of V(b)/V(a) to V(b)/V(a) you will be looking at the socalled "loop gain". For a little bit about the loop gain idea, see Figure 1 of this Wikipedia article for a block diagram. You have the openloop gain, which is called A_{OL} in that diagram. Then there's the feedback factor B, which, if C4 were not present, would be R12/(R12+R11) = 1/34 in your circuit (assuming C5 is a short). The openloop gain A_{OL} is approximately the ratio of the AC output voltage to the voltage difference between the noninverting and inverting inputs of the input diff pair. Here's the short description of stability theory. The closedloop amplifier gain A_{CL} in Figure 1 of the Wikipedia article is: A_{CL}=A_{OL}/(1+A_{OL}B) Now the product A_{OL}B is a complex number, having amplitude and phase. Suppose the amplitude of A_{OL}B were 1 and its phase were +/ 180 deg at some frequency f0. That's the same as the real number 1. Then the denominator of the expression above for A_{CL} would be zero, giving infinite gain at the frequency f0. But that's what an oscillator is. So you want the phase of A_{OL}B to be far away from +/180 deg at the frequency for which the magnitude of A_{OL}B is 1 (same thing as 0 dB). The difference between the actual phase shift of A_{OL}B at the frequency for which its magnitude is 0 dB, and the "drop dead" value of 180 deg is called the phase margin. Your circuit has a problem because its loop gain begins to flatten out above 100 kHz. That's caused by C4. Try removing C4, or setting it to some silly value like 0.01 pF, and plot V(b)/V(a) (note the minus sign). Find the frequency at which its magnitude is 0 dB. That's called the unity loop gain frequency. Then look at the phase at this frequency. To pick a number, assume it's 120 degrees. That will give a phase margin of 60 degrees. To prevent overshoot in the transient response, shoot for a phase margin of 80 degrees. This may involve tweaking the input stage emitter degeneration or the Miller comp cap or both. To look at actual data points on the graph, leftclick on the graph label text at the top (the V(b)/V(a)). This will give you a cursor that you can drag and read the amplitude in dB and phase in degrees. Edit: Just to clarify, if you plot V(b)/V(a), that is the same as A_{OL}B, which is the socalled loop gain. 
22nd November 2008, 03:40 AM  #140 
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