Frequency response of MOS amplifiers

 

For high frequency signals

No negligible admittances of parasitic capacitances

Gains and input and output impedances are functions of signal frequency

High-frequency small-signal equivalent circuit of a MOSFET must be used

NMOS single-ended amplifier with enhancement-mode load device

 

 

Using the high-frequency small-signal equivalent circuit the Miller effect will be taken into account

Simplified equivalent circuit of MOS gain stage valid in high frequency range:

 

 

 

Corresponding transfer function:

 

Positive real zero:

s_z = g_m1 / C_gd1

 

Negative poles:

s_p1 = -G_s / C_in

s_p2 = -G_Leq / (C_Leq + C_gd1)

Frequency response:

By replacing s by jw

 

if |s_p1| << |s_p2| and s_z :

3 dB frequency (where A_v(jw) = 1/2)

w_3dB ~ |s_p1| = G_s / C_in

 

If high gain is required:

• Ratio (W/L)₂ must be small
• W limited by the fabrication process
• Large L (long load device) is required
• Consequently C_Leq is large
• Frequency response is degraded