MOSFET Small Signal Model

MOSFET SMALL SIGNAL MODEL

The small signal model of MOSFET can be obtained similar to JFET. Fig.2.23 shows the small signal equivalent circuit of MOSFET.

Since gate is insulated from channel by gate-oxide, input resistance of transistor is infinite.

i. Small signal parameters are controlled by the Q-point. For the same operating point, MOSFET has lower transconductance and an output resistance that is similar to the BJT.

ii. In saturation mode, MOSFET acts as a voltage controlled current source. The control voltage is V_GS an the output current is i_D.

STEPS INVOLVED IN SMALL-SIGNAL ANALYSIS OF MOSFET

Step 1: Complete a DC analysis

The goal of this DC analysis is to determine

(a) the dc voltage VGS for MOSFET

(b) the de voltage VDS for MOSFET

Step 2: Calculate the small signal circuit parameters for the MOSFET.

Step 3: Replace all MOSFETs with their small-signal circuit model.

Step 4: Set all dc sources to zero.

Step 5: Analyze small-signal circuit.

MOSFET COMMON SOURCE SMALL-SIGNAL AMPLIFIER ANALYSIS

The common source MOSFET amplifier is shown in Fig.2.24.

Construction

C_C1 and C_C2 are the coupling capacitors and the capacitor C_S is bypass capacitor. The capacitance values are chosen to be large so that their reactances are very small at the operating frequency.

Analysis

Fig. 2.25 shows the small signal equivalent circuit for the common source MOSFET amplifier.

We will calculate the input and output impedances, current and voltage gain similar to BJT and FET.

Input Impedance

From the Fig.2.25, and noting that i_g = 0

Output Impedance

To calculate the output impedance, we get V_S = 0

When V_S = 0 ⇒ g_m V_GS = 0

The input impedance of the dependent current source is infinite. Thus output impedance is written as

Voltage Gain

Overall Voltage Gain  

Applying voltage division rule

Substitute R_in = R_G from input impedance (1)

Substitute (4) in (3)

Substitute (2)

Current Gain

Using current division rule

At the input, 

Substitute (8) in (7)

When R_G → ∞, A_V = ∞

The equation (9) gives the overall current gain of CS amplifier.

Properties

i. High input impedance

ii. High output impedance

iii. Relatively high small-signal voltage gain

iv. Very high small-signal current gain