Input and Output Resistance

1. EC8452
ELECTRONIC
CIRCUITS II
Dr.N.G.PRAVEENA
ASP/ECE
Voltage Series Feedback

2. BASIC FEEDBACK
CONCEPT

3. Topologies of Feedback Amplifiers
 Based on the type of sampling (voltage/current) at
the output side
 Based on the type of mixing (series/shunt) to the
input side
1.Voltage-series feedback or series-shunt feedback
2.Current-series feedback or series-series feedback
3.Current-shunt feedback or shunt-series feedback
4.Voltage-shunt feedback or shunt-shunt feedback

4. Voltage-series feedback or series-shunt
feedback
 Voltage Amplifier.
 VCVS
 Av =
𝑽𝑶
𝑽𝒊
 β =
𝑽𝒇
𝑽𝒐
 Avf =
𝑽𝑶
𝑽𝒔

5. Input Resistance with feedback(𝑹𝒊𝒇)
 𝑅𝑖𝑓 =
𝑉𝑠
𝐼𝑖
Apply KVL at the input circuit
𝑉
𝑠 = 𝑉𝑖 + 𝑉𝑓
𝑉
𝑠 = 𝑉𝑖 + β𝑉
𝑜
𝑉
𝑠 = 𝑉𝑖 + β𝐴𝑣𝑉𝑖
𝑉
𝑠= 𝑉𝑖(1 + β𝐴𝑣)
𝑉
𝑠= 𝐼𝑖𝑅𝑖(1 + β𝐴𝑣)
𝑹𝒊𝒇 =
𝑽𝒔
𝑰𝒊
= 𝑹𝒊(𝟏 + 𝜷𝑨𝒗)
𝑹𝒊𝒇 >> 𝑹𝒊

6. Output Resistance with feedback(𝑹𝒐𝒇)
 REMOVE RL
 REMOVE SOURCE (Voltage source – Short Circuit, Current
Source – Open Circuit )
 CONNECT VOTLAGE SOURCE
 Apply KVL to the output circuit,
V = I𝑹𝒐 + 𝐴𝑣𝑉𝑖 --------------- (1)

7. Output Resistance with
feedback(𝑹𝒐𝒇)
 We know that
𝑉
𝑠 = 𝑉𝑖 + 𝑉𝑓
𝑉𝑖 = − 𝑉𝑓 (𝑉
𝑠 = 0)
= -βV ------(2)
Sub (2) in (1)
V = I𝑅𝑜 − βV𝐴𝑣
V (1 + β𝐴𝑣) = I𝑅𝑜
𝑹𝒐𝒇 =
𝑽
𝑰
=
𝑹𝒐
(𝟏 + 𝜷𝑨𝒗)
𝑹𝒐𝒇 ≪ 𝑹𝒐