Circuit Wizards

[lacy]

I have a couple of questions about the Circuit Wizards.

Is there a detailed explaination for the CE-BJT Wizard and how it comes up with the values, resistors, etc.? The help file give this information, but it is a little hard to follow and lacks some details. The way this wizard creates it's circuit is different than the way I have been taught how to do this and if I had a more detailed explaination it might help me get my brain around it. Just point me to some reference material concerning this and I would be a very happy camper.

I have 2 questions concerning the Mono-Stable 555 Timer Wizard.

1. Why have an input signal there? Most of the literature I have read when calculating the timing, the input signal doesn't figure into it since the input is usally going to be a short pulse with an extremely long period  (usually longer than the time-out period) or just a switch to ground. 

2. With ithe current configuration the time-out period seems limited to approx 15-16 minutes as I have observed  Is this correct? Is there anyway to get the Wizard to go any longer? I have seen delays up 1 hour on these timers. I know I can change the values once the circuit is created, but I shouldn't have to do so. That is why they're there so I won't have to do things maually.It is my opinion that the input signal being there is messing up the calculations. We should have a way of turning it off.

Thanks for any information anyone may have on this.

[lacy]

Bumping up so it will be answered.

[kittmaster]

Most people can't calculate the circuit by itself. I think if they tried to explain it in more detail it would make the help files blow up rapidly.

Most astable operations of a 555 in the hour range becomes unpredicatble as the larger the capacitance value goes up, the less accurate the dielectric and leakage increases. They are assuming that most people will not wait 1 hour on the simulation to see what the final results are.

I do agree however that it should calculate any value provided and not be fetered with time limits

[lacy]

I can understand what your saying about the timer. From what I have read (if that is accurate) some of the newer timers are fairly stable up to 1 hour. But I assume they erred on the side of caution when they designed the wizard. Thats o.k I can work around that.

As far as the BJT Wizard. I was trained to calculate the DC bias by assuming a collector current below max Ic and a VCE of approx half of the supply voltage and then figuring my resistors from that point. None of the text I have encountered have the input signal in its calculation though. Is the wizard figuring something different? Is this for extremely small signal applications like say in the uV range? I am missing something in understanding this and I am not sure what that something is. You wouldn't by chance know of a tutorial about this on the internet that may help.

Thanks for your answer to my question.

Message Edited by lacy on 08-28-2007 09:43 PM

[Euler's_Identity]

Lacy,

Concerning the 555 one shot, as a guess, perhaps, since MultiSim is all about schools, the idea was to make one aware of rise and fall times, produce a realistic scenario.  For a 555 to function properly, the input trigger must return high.  One cannot send a negative step and expect the output pulse to be a pulse.  (I slipped in that hole once myself.)  It must be a negative input pulse.  Hence, if the negative pulse either doesn't return high (a step, not a pulse) or doesn't return high fast enough (indeterminant w/o a Schmitt trigger first,) the 555 will give, likely, undesirable results.  I'd rather see realistic results BEFORE I used the parts and got an unexpected surprise.

Maybe that's what it's about.

[lacy]

Thanks for your thoughts on this subject. I understand the reason behind it, but a majority of the time the mono-stable is used in very short negative input pulses with long intervals (longer than the time-out period of the timer circuit). Most of my mono-stable designs either have a switch to ground with a resistor attached to positive or a transistor integrator circuit for creating the short negative going pulse. The circuit wizard doesn't allow you to turn off the input signal but only to run the frequency down to minimum allowed. This, in my opinion, is causing the time-out period to be limited to only 15-16 minutes. We should have an option to turn it off or on is all I am saying depending on your design application.

You wouldn't by chance have any resources I could refer to concerning the BJT_CE Wizard?

[Euler's_Identity]

Sorry, I don't have anything on the CE (Common Emitter) BJT (BiPolar Junction Transistor) circuit wizard.  I just know generally about the CE setup.

As I remember it, and it's been a while, the input resistance is Rb1||Rb2||Bre, where Bre is the base input resistance, re=25mV/Ie (at room temp) times B (Beta).  The input coupling C should be set to allow the lowest output frequency to pass to the amp.  (For low power devices, it's generally safe to use B=100.)  The DC point at the collector will be Vcc-IcRc, where Ic is a fraction lower than Ie, and Ic=Ie.  The bypass cap at the emitter is used to create an AC short at the emitter, at the lowest frequency of interest, so as to make the gain largely independent of Re.  The ouput coupling cap should be chosen so that the lowest frequency of interest can pass to the load.  The output resistance, Rc, then forms a parallel combination with the load (to ground), assuming the ouput cap is a short.  I'm thinking you'd want to have the maximum swing, so you'd probably want Vc near Vcc/2.  I think the gain is something like A=Rc/2Re or something like that, given proper C choices.  The reason Re exists is to provide emitter feedback and make for a more temperature stable operating point; it also raises the input resistance.

The above may help, or then again, it may just speak volumes about how little I remember.

Also, one last tidbit is that the CE, CB and CC (follower) configurations are all the same circuits, just with rearanged inputs, outputs, and power sources.

    

[lacy]

Thank you for the information that you provided for the CE-Wizard. It sound like we both design CE Amps pretty much the same. If you try the CE wizard, you will find that it will not let you set the VCE to VCC/2 for maximum swing. When you do this it changes the Ic and other variables and then coughs up an error. I tried designing a CE amp at 12V VCC 1Khz input a 1mV with a source resistance of 100 ohms and an output load of 5K. Cutoff frequency was 100 Hz. collector IC was set to 1mA When I set VCE to 6 VDC It changes the Ic and then says something to the effect "your setting will cause Re to be less than zero Please adjust either VCE, IC, or Maximum ouput swing" or something to that effect.

This is where I think it is confusing me, because like I said I select a Ic somewhere below maximum usially in the 1-10mA range Set VCE to VCC/2 and then figure my Collector resistor, then my emitter resistor (usually 25% VCC and assuming Ic=Ie) then I proceed to figure my base resistors based upon Ic/Beta*10 for the lower resistor and IC/Beta*10 +Ie for the upper resitor. I get my Base voltage from VBE (.700)+Ve. Now all of this is just for DC operating point. Usually if these calculation go well,and as long as your signal doesn't range higher that the Ic and VCE you have selected your circuit should work and give you the gain your looking for. But this thing has totally blown my mind as to how it is figuring its component values.

As a footnote to this, I have another circuit wizard program. I manually figure my resistors just to test it. I then plug in my desired gain, VCC, etc. and it comes pretty close to what I have manually figured on paper. 

[Euler's_Identity]

"I tried designing a CE amp at 12V VCC 1Khz input a 1mV with a source resistance of 100 ohms and an output load of 5K. Cutoff frequency was 100 Hz. collector IC was set to 1mA When I set VCE to 6 VDC It changes the Ic..."

The circuit looks like:

Vcc ---- Rc ---- Vce ----- Re ----- GND

Vce is not Vc.  You want Vc at Vcc/2, where Vc=Vce+Ve.
 
Could this be your problem?

[lacy]

That may be it entirely. I am getting my terms confused.   I will try that and see if it makes a difference. Thanks so much. This may be the missing link I have been looking for. I really appreciate your information and time to respond to my inquiry.