ENVLP

A back-of-the-envelope symbolic circuit analyzer PositiveFB 

        
        

          
          
          
        

      


      


      

        

          
          
          
          
        

        

      

    

  


  


  






  

    
ENVLP

    
    
  


  

    
Getting started

    
    
      
  1. Place the parts. Click a part in the palette on the left, then
        click the canvas to drop it. The letter keys do the same: R resistor,
        C capacitor, L inductor, V voltage source, B current source,
        O op-amp, M MOSFET (press M again for the P-channel), Q BJT (again for
        the PNP), G ground, Shift+G supply rail, D diode, T transformer,
        I for the full list. While you are placing, Space rotates the part
        and Shift+Space flips it; Esc cancels.
    2. 

      
  2. Wire them up. Press W, click a pin or any grid point to
        start a wire, then click the destination. Every click commits a
        segment and the wire carries on from where it stopped. Space swaps
        the route between horizontal-first and vertical-first, Enter ends the
        wire, Esc cancels it. A wire that ends on another wire's endpoint
        becomes one continuous wire; ending in the middle of another leaves
        a junction dot. Moving a part takes the wires attached to its pins
        with it, and a wire can be dragged: the segment under the cursor
        moves sideways and the rest of the route stretches to follow.
    3. 

      
  3. Name the nets. Press N, type one or more names separated by
        spaces, then click each net in turn. Click a name to select it and
        press Del to remove it.
    4. 

      
  4. Ground the circuit. At least one GND symbol is required. The
        supply rail (VDD) is optional; its voltage is an operating-point
        setting, not a value on the symbol.
    5. 

      
  5. Run an analysis. Pick one on the right, fill in the fields it
        asks for, and press Run analysis (Ctrl+Enter). The left pane
        shows the result typeset (Math) and as text (Report); the right pane
        shows the Bode, Nyquist, Nichols or noise plot.
    6. 

      
  6. Adjust the parts. Click a part to edit its value, its model
        parameters (gm, ro, Cgs, and the passives' parasitics) and its
        copies. Each model parameter has its own switch: a transistor's
        capacitors are off until you turn them on, and a passive is ideal
        until you give it a stray capacitance or a winding resistance.
    7. 
    

    
Drag the thin dividers between the panels to resize
      them.

  


  

    
Ignoring negligible terms

    
ENVLP reports the low-entropy form of a result: every term is
      ranked by its worst-case magnitude over the sweep, and the terms far
      below the dominant one are dropped. The answer then reads like the
      expression a designer would write by hand instead of a page of
      negligible products.

    
Two thresholds decide what counts as negligible. They live in the
      negligible terms section under the analysis options:

    

      
Component value threshold

      
A series or parallel pair collapses when one element is at least
        this many times the other. 10 (which is 20 dB) means a 10k in
        parallel with a 1k is reported as the 1k.

      
Pole / zero threshold

      
A pole or a zero is dropped when it is more than this many times
        the reference frequency. A pole above the top of the sweep is
        dropped as well — it lies outside the analysed band.

      
Reference

      
What the pole/zero threshold is measured against: the
        dominant pole / zero, or the unity-gain bandwidth (the
        latter drops everything beyond a multiple of the crossover
        frequency).

    

    
Ignore negligible terms switches the whole reduction on and
      off; with it off you get the exact, fully expanded expression.
      Approx roots is independent: it lets a factor that cannot be
      factored exactly be given by its numeric roots instead.

  


  

    
Keyboard shortcuts

    

      
R C Lresistor, capacitor, inductor

      
V Bvoltage source, current source

      
M / Shift+MN-MOSFET / P-MOSFET (press M again to switch)

      
Q / Shift+QNPN / PNP (press Q again to switch)

      
Ddiode

      
Oop-amp

      
G / Shift+Gground / supply rail (VDD)

      
T Ktransformer, coupled inductors

      
Icomponent menu (every part)

      
Wwire tool

      
Nplace net label(s)

      
Spacerotate the part being placed or the selection by 90°
        (while wiring: swap the route)

      
Shift+Spaceflip horizontally

      
Ctrl+Spaceflip vertically

      
Ffit the components to the view

      
Deldelete the selection

      
Enterend the wire being drawn

      
Esccancel the current action

      
U / Shift+Uundo / redo

      
Ctrl+Z / Ctrl+Yundo / redo

      
Ctrl+Enterrun the analysis

      
F1 / ?this help

    

    
Shortcuts are ignored while you are typing in a field.

  


  

    
What each analysis does

    

      
Transfer function H(s)

      
The small-signal gain from the input source to the output, as a
        rational function of s. It reports the DC gain, the pole and zero
        corner frequencies, the dominant time constant and the factored
        form of the result.


      
AC (small-signal)

      
The small-signal response of the whole circuit: every independent
        source is excited with its own AC value and the responses are
        superposed. There is no input source to name, so the analysis has no
        in field.


      
DC operating point

      
The large-signal bias of the circuit: the voltage at every node and
        the operating point of every device (its drain current and its
        saturation voltage). The device model, the supply rail and the
        threshold voltages are set in the operating point section,
        which belongs to this analysis alone.


      
PSR / PSRR

      
How much of the ripple on the supply rail reaches the output: the
        power supply rejection, referred to the output.


      
Loop gain (return ratio)

      
The return ratio of the feedback loop, by the Rosenstark method.
        Name the amplifier in ref. amplifier: it is the part whose
        gain is set to zero to break the loop. The report gives the
        asymptotic gain, the return ratio, the feedback factor and the
        closed-loop gain.


      
Short-circuit current

      
The current that flows into the output node when it is shorted to
        ground. The short is the excitation, so this analysis has no input
        source to name either.


      
Input impedance

      
The impedance seen by the input source: the voltage across it per
        amp of drive. It is the source's own response, so it needs the input
        source and no output.


      
Output impedance

      
The impedance seen looking back into the output, with every source
        turned off. It needs the output node and no input source.


      
Noise

      
The output noise density and the input-referred noise, integrated
        over the sweep band, with the contribution of each source broken
        out.


      
Differential (Adm / Acm / CMRR)

      
The differential gain, the common-mode gain and the common-mode
        rejection ratio of a differential pair. The input is a differential
        port (in+ and in−) and the output is a port too, given
        as two nodes.

    

  




  

    
Components

    
    
  

  




  

    
Place net label(s)

    
    
  

  

    
Type one or more net names, separated by spaces. Each name is placed on
       the next net you click; Esc stops.