Series–Series Feedback Explained | Transconductance Amplifiers, Loop Gain & Resistance Effects

In this lecture, we explore the Series–Series feedback topology—the fundamental feedback configuration used in transconductance amplifiers (voltage-to-current stages).
We develop the intuition, derive the equations, and analyze how feedback reshapes gain, input/output resistance, linearity, and stability.

Key Topics Covered:
• What “Series–Series” feedback means
• Why it is the natural topology for transconductance amplifiers (gm stages)
• Closed-loop gain: Af = A / (1 + Aβ)
• How feedback increases input resistance and increases output resistance
• How gain becomes highly predictable under large loop gain
• Computing the feedback factor β by current sensing at the output
• Understanding loading effects (R11, R22) and how they alter the open-loop model
• Constructing the complete loop-broken equivalent circuit
• Full worked example with an op-amp + PMOS stage:
– computing β, R11, R22
– deriving open-loop gain
– obtaining closed-loop gain and resistance transformations
• Practical interpretation: when and why Series–Series is preferred over other topologies (Series–Shunt, Shunt–Shunt, Shunt–Series)

By the end of this video, you will be able to:
• Identify Series–Series feedback in real analog circuits
• Compute β for current-sensed networks
• Predict how feedback shapes Rin, Rout, and gain
• Analyze transconductance amplifiers using rigorous loop-gain reasoning
• Build intuition about how analog designers choose feedback structures for precision and robustness

This lesson is ideal for students learning Analog Electronics, Microelectronics, and VLSI Design, as well as practicing engineers revisiting classical feedback theory with modern CMOS intuition.

Видео Series–Series Feedback Explained | Transconductance Amplifiers, Loop Gain & Resistance Effects канала Abhijit Pethe