2022-10-09 15:06 Status: # Common Gate Amplifier aka [[cascode]] Best thought of as an impedance transformer (like the [Common Drain Amplifier](Common%20Drain%20Amplifier.md)). Gate is connected to a constant voltage. Input is the current of the source, while the output is the current of the drain. Commonly a load resistance.  ## Input and Output Resistances  Bulk connection is absorbed in the gate because the two connections act in parallel (fiddly - look at as an exercise).  This is approximately equal to $\frac{1}{g_{mx}}+\frac{R_L}{g_{mx}r_{ds}}$  The common gate amplifier reduces the load resistance by the intrinsic gain: A large load resistance is the load resistance divided by the gain. For a small load resistance, the smallest input impedance you can have is $1/g_{mx}$ - effective transconductance. **This is important!** The smallest impedance you can get on a chip is $1/g{m}$ and the best that your transistor can do is to decrease the load by the intrinsic gain.  Think of the common gate amplifier as improving the input and output impedances by the intrinsic gain. ## Current Gain $A_I=\frac{I_{out}}{I_{in}}=1$ since no current flows into the gate or the body. ## PMOS Common Gate  ## Application Increase output impedance of a transistor. ### Example: [Cascode](cascode.md) Current Mirror Finite output resistance can effect the performance of a current mirror. Adding a common gate amplifier does not effect the current but increases the output impedance, making it more like an ideal current source!   ### Different method for [cascode](cascode.md) current mirror  Uses up alot of voltage - usually not possible with supply voltages available. ## [Cascode](cascode.md) Gain Stage Ensure that both the upper and lower branch are cascoded.  Active load has a [cascode](cascode.md) added to it. ## Output Resistance  If we assume all transconductances and drain-source resistances are equal, $r_{out}=\frac{1}{2}r_{ds}^2g_m$ which is even better than 2 common-source amplifiers cascaded (slightly higher gain). It is important to [cascode](cascode.md) **both branches**. E.g. if no [cascode](cascode.md) on active load (p-type current mirror) the total gain is the intrinisic gain of the bottom transistor - only factor of 2 improvement. [^1] --- # References [^1]: [vr-4602-wk04-sc04-commongate](../../Spaces/University/ELEC4602%20–%20Microelectronics%20Design%20and%20Technology/Lectures/W4/vr-4602-wk04-sc04-commongate.mp4)