Integrated Circuits and Systems group, IIT Madras

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courses:ee3703_2017:invchar [2017/01/10 09:36] (current)
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 +====== 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. ​