Activity: The Source follower (NMOS) - ADALM1000

Objective:

To investigate the simple NMOS source follower amplifier also sometimes referred to as the common drain configuration.

Notes:

As in all the ALM labs we use the following terminology when referring to the connections to the M1000 connector and configuring the hardware. The green shaded rectangles indicate connections to the M1000 analog I/O connector. The analog I/O channel pins are referred to as CA and CB. When configured to force voltage / measure current -V is added as in CA-V or when configured to force current / measure voltage -I is added as in CA-I. When a channel is configured in the high impedance mode to only measure voltage -H is added as CA-H.

Scope traces are similarly referred to by channel and voltage / current. Such as CA-V , CB-V for the voltage waveforms and CA-I , CB-I for the current waveforms.

NMOS Source Follower:

Materials:

ADALM1000 hardware module
Solder-less breadboard
Jumper wires
1 - 1.5 KΩ Resistor (R_L)
1 - small signal NMOS transistor (enhancement mode CD4007 or ZVN2110A M₁)

Directions:

The breadboard connections are shown in figure 1. The output of the channel A generator, is connected to the gate terminal of M₁. The drain terminal is connected to the positive (+5V) supply. The source terminal is connected to both the 1.5 KΩ load resistor and Scope channel CB-H. The other end of the load resistor is connected to dround.

Figure 1 Source Follower

Hardware Setup:

The channel A voltage generator should be configured for a 100 Hz Sine wave with 3 volt Max and 2 V Min. The channel B scope in Hi-Z mode CB-H is used to measure the voltage at the source. To measure the input to output error or offset, the CA-V - CB-V Math waveform can be displayed. To measure the input to output gain, the CB-V / CA-V Math waveform can be displayed.

Procedure:

The incremental Gain (Vout /Vin) of the source follower should ideally be 1 but will always be slightly less than 1. The gain is generally given by the following equation:

From the equation we can see that in order to obtain a gain close to one we can either increase R_L or decrease r_s. We also know that r_s is a function of I_D and that as I_D increases r_s decreases. Also from the circuit we can see that I_D is related to R_L and that as R_L increases I_D decreases. These two effects work counter to each other in the simple resistive loaded emitter follower. Thus to optimize the gain of the follower we need to explore ways to either decrease r_s or increase R_L without effecting the other. It is important to remember that in MOS transistors I_D = I_S ( I_G = 0 ).

where K =μ_nC_ox/2and λ can be taken as process technology constants.

Looking at the follower in another way, because of the inherent DC shift due to the transistor's V_th, the difference between input and output should be constant over the intended swing. Due to the simple resistive load R_L, the drain current I_D increases and decreases as the output swings up and down. We know that I_D is a (square law) function of V_GS. In this +4V to +2V swing example the minimum I_S = 2V / 2.2KΩ or 0.91 mA to a maximum I_S = 4V / 2.2KΩ or 1.82mA. This results in a significant change in V_GS. This observation leads us to the first possible improvement in the source follower.

The current mirror from activity 6M is now substituted for the source load resistor to fix the amplifier transistor source current. A current mirror will sink a more or less constant current over a wide range of voltages. This more or less constant current flowing in the transistor will result in a more or less constant V_GS. Viewed another way, the very high output resistance of the current source has effectively increased R_L while r_s remains at a low value set by the current.

Additional Materials:

1 - 1 KΩ Resistor
1 - small signal NMOS transistor (M₁ZVN2110A)
2 - small signal NMOS transistors (M₂, M₃CD4007)

Figure 2 Improved Source Follower

Source follower output impedance

Objective:

An important aspect of the source follower is to provide power or current gain. That is to say drive a lower resistance (impedance) load from a higher resistance (impedance) stage. Thus it is instructive to measure the source follower output impedance.