====== Inverter characteristics ======
===== Goals =====

  * You'll be using inverters in a CD4069 CMOS inverter chip as transconductors(VCCS). In this experiment you'll measure the large and small signal characteristics of this transconductor. In all experiments, remember to disable the unused inverters. Do all steps for V<sub>dd</sub>=6V for the inverter unless otherwise specified. Choose the supply voltages  V<sub>cc</sub> and  V<sub>ee</sub> of the opamp appropriately.
    * Determine the large signal I<sub>out</sub> versus V<sub>in</sub> 
    * Determine the large signal V<sub>out</sub> versus V<sub>in</sub>
    * Determine the small signal parameters of the transconductor

===== References =====
  * View the [[http://www.ee.iitm.ac.in/~nagendra/EC330/200801/lectures/ec330-intro/ec330-intro.swf|lecture]]
===== Experiments =====

{{invchar1.png?400}}

  * Determine I<sub>out</sub> versus V<sub>in</sub> of the transconductor using the circuit above. Vary the input voltage from 0 to Vcc. You can use a separate power supply for generating the variable bias voltages or a resistive voltage divider(say, a chain of 16 identical resistors from Vcc to ground). What is the self bias voltage V<sub>M</sub> of the inverter? Estimate the transconductance at V<sub>M</sub>. ** Choose the supply voltages for the opamp appropriately so that it doesn't saturate **.

{{invchar2.png?200}}

  * Determine V<sub>out</sub> versus V<sub>in</sub> of the transconductor(with an open circuit load as shown above). Take more points in the high gain region for accurate determination of gain. 

{{invchar3.png?300}}

  * Bias an inverter at V<sub>M</sub> by shorting its input and output as shown above. What is the small signal resistance looking into this circuit? Measure it by forming a voltage divider with one arm being the small signal resistance as shown in the circuit above. Are the values consistent with the earlier measurements? Repeat this measurement for V<sub>dd</sub>=5V and V<sub>dd</sub>=9V. These will be useful for future experiments.

===== Exercises =====

  * From the above measurements, estimate the small signal output resistance Rout of the transconductor, and threshold voltages and current factors of nMOS and pMOS transistors.

  * You'll need the measured parameter values for subsequent experiments(g<sub>m,inv</sub>, r<sub>out,inv</sub>, V<sub>Tp</sub>, V<sub>Tn</sub>, μ<sub>p</sub>C<sub>ox</sub>W<sub>p</sub>/L<sub>p</sub>, μ<sub>n</sub>C<sub>ox</sub>W<sub>n</sub>/L<sub>n</sub>). So keep a record of them.

===== Applications =====

  * Transconductors are building blocks of practically all analog ICs albeit with numerous other topologies besides the CMOS inverter. Regardless of the topology, you'll need to do similar simulations/measurements as the ones here to ascertain their parameters.