1 Single N-FET or P-FET vs Complimentary N-P-FET Input Stage Limitations

The common-mode voltage range of an amplifier is the range of usable input voltages allowing for linear operation. Depending on the input stage topology, amplifiers may have common mode input range (V_CM) which may be limited relative to one or both supply rails; best case scenario is achieved when input voltage range extends slightly beyond both supply rails (rail-to-rail operation). Limitations in the V_CM range come from operating voltages needed to bias the transistors in the input stage, and to ensure operation within a linear range (saturation range for MOSFET or active range for bipolar transistors). We’ll illustrate these limitations for MOSFET amplifier input stages. Figure 1-1 shows a simplified representation of an N-channel MOSFET input stage. The stage consists of a current source (single NMOS, Q3, shown for simplicity), a differential pair with input voltage applied to the gate of each transistor, and an active load PMOS current mirror, Q4 and Q5. The NMOS differential pair has an input common mode voltage limitation with respect to the negative rail, –V_s. Performing Kirchhoff’s voltage walk from –V_s to V_in+, we have:

Therefore, the N-channel MOSFET V_CM range is limited to -V_s by a certain voltage as detailed in Equation 1. Note that we make some important assumptions, namely that V_sat of both NMOS and PMOS transistors are perfectly matched and equal to 0.1V (a typical value). Similarly, we assume that V_gs for both NMOS and PMOS transistors are equal and have a value of 0.9V, a typical value to turn on the transistors. With these assumptions in mind, we can say that this simplified NMOS input stage allows input common mode voltage range operation about 1 V from V_s. Performing Kirchhoff’s walk from the opposite end, +V_s to V_in±, we get:

Given that V_gs typically exceeds V_ds for a MOSFET at the edge saturation (V_sat at a minimum, V_gs at a maximum), the maximum V_CM comes within V_sat, or 100 mV, of the positive rail, +V_s.

Conversely, the P-channel MOSFET input stage is limited on the positive side, usually on the order of 1 V from positive rail, +V_s. On the negative side, the common mode voltage range of a P-MOSFET can come within V_sata, or 100 mV from the negative rail,-V_s.

To avoid the limitations of the single differential pair input stage, a complimentary N-channel and P-channel MOSFET (CMOS) input stage design can be used. This design uses two input differential pairs (an N-channel MOSFET pair and a P-channel MOSFET pair), a current steering scheme, and a double-folded cacose summing the two input signals (Figure 1-2). V_set is a voltage source used to control the functionality of the diverting transistor Q₈. For common mode voltage below +V_s – V_set, Q₈ is off, and the drain current (I_d) from Q₅ (current source) flows straight through the P-channel differential pair (Q₁ and Q₂). The double-folded cascode allows the drains of Q1 and Q2 to be biased down to V_sat above – V_s, resulting in V_CM swing below the negative rail. This allows the V_CM to extend a certain voltage, ΔV_P, below the negative rail. Similarly, for common mode voltage above +V_s – V_set Q₈ is on, and I_d is steered from the P-channel pair to the N-channel pair via the current mirror (Q₆ and Q₇). The V_CM range can therefore exceed the positive rail, +V_s, by a certain voltage, ΔV_N. To sum it up, this gives an op-amp with this input stage topology a rail-to-rail V_CM range as detailed in Equation 3.

Now that we understand the rail-to-rail operation of a complementary input stage, we more elaborately address ΔV_N and ΔV_P. Looking deeper into the complementary input stage amplifier in Figure 1-2, we can see that the rail-to-rail input performance is dependent on the second stage. Using Kirchhoff’s voltage law from the positive rail down to the input, similar to the one performed previously on Figure 1-1.

We can see from Equation 4 above that the complimentary input stage amplifier has input common mode range 0.7 V above the positive rail. We can find the common mode input voltage range to the negative rail by performing the same procedure.

From Equation 4 and Equation 5 we find that the common mode input voltage range extends beyond the positive and negative rail typically by about 0.7 V, which is represented by the aforementioned terms ΔV_N and ΔV_P. In data sheets, you will notice that most rail-to-rail amplifiers are specified up to 0.1 V (not 0.7 V) beyond the supplies. This is due to the protection diodes between the input and each rail.