ElectronicsI_L14 - Lecture 14 Voltage Gain We can calculate...

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Lecture 14
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Voltage Gain • We can calculate the voltage gain by looking back to what is happening in the circuit: • The instantaneous drain voltage is: • Which is, under the small signal condition: • In terms of the DC set point output voltage: • Thus the signal component is: • We can then write the gain as: D D DD D i R V v = ( ) d D D DD D i I R V v + = d D D D i R V v = D gs m D d d R v g R i v = = D m gs d V R g v v A =
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Example 2 • We want to design an NMOS amplifier to provide a 0.5V peak output signal across a 50k load, which can be used as the drain resistor • If a gain of at least 5V/V is needed what should g m be? •I f V DD =3V, what values of I D and V OV might be used?
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What value of g m is needed? We need to solve the gain equation for the transconductance: What value of I D do we want to choose? – Start with a 1/2 rule: We only have two components, so we want 1/2 of the voltage to drop across the load – This gives us the current to shoot for: How about the overvoltage? Can we choose any? As the problem is stated, we don’t have sufficient information to use the device physics to choose a value But we do have a specified output amplitude, from which we can back out the input voltage swing. We’d like this to be much less than the overdrive voltage. Let’s say <=10% V mA k V V R A g D V m / 1 . 0 50
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ElectronicsI_L14 - Lecture 14 Voltage Gain We can calculate...

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