====== Charge pump -ve voltage generator ======
===== Goals =====
  * Design a negative voltage generator from a positive supply. 
  * Design a circuit to get a higher voltage than the available supply



===== Preliminaries =====
{{vnegchargepump.png?200}}

  * Analyze the circuit in the figure above. Determine the waveforms V<sub>A</sub> and V<sub>B</sub> in steady state. This requires a careful evaluation of the region of operation of the MOS transistors. 

===== Experiment =====

{{vnegchargepump1.png?300}}

  * Transistors Mn1 and Mn2 are added to multiplex V<sub>A</sub> and V<sub>B</sub> and result in V<sub>out</sub>. Implement this circuit using CD4069 inverters. __Think carefully about which terminal goes where!__ You'll need to use separate ICs for the inverter and the rest of the circuitry because, on a chip, the supply and ground pins are all shorted together. You can use clock frequencies in the tens of kHz range, capacitors in the 10nF range, and a supply of 5V. 

  * What is the output voltage V<sub>out</sub> with no load(C<sub>L</sub>)? and with a constant current load I<sub>L</sub>? 

  * Build and demonstrate a circuit that gives a positive output of approximately twice the power supply wrt ground.

===== Applications =====

  * This is useful when a dual supply is required to be generated from a single supply, such as a battery. Such circuits are used in audio amplifiers which, when operated from a dual supply, have a quiescent output of 0V and hence don't need a bulky ac coupling capacitor. These circuits are also used when a small portion of the IC needs a higher supply voltage, but multiple power supplies can't be used. Examples are flash memories(USB sticks) and EEPROMS which operate from a low voltage supply(3V-5V in a mobile phone/MP3 player) but need a high voltage(~ 15V) for write/erase.