7.4.1 Input and Output Stage

The rail-to-rail input stage of this family provides more flexibility for the designer. The LMV93x-Q1-N use a complimentary PNP and NPN input stage in which the PNP stage senses common-mode voltage near V⁻ and the NPN stage senses common-mode voltage near V⁺. The transition from the PNP stage to NPN stage occurs 1 V below V⁺. Because both input stages have their own offset voltage, the offset of the amplifier becomes a function of the input common-mode voltage and has a crossover point at 1 V below V⁺.

Figure 29. Simplified Schematic Diagram

This V_OS crossover point can create problems for both DC− and AC-coupled signals if proper care is not taken. Large input signals that include the V_OS crossover point will cause distortion in the output signal. One way to avoid such distortion is to keep the signal away from the crossover. For example, in a unity gain buffer configuration with V_S = 5 V, a 5-V peak-to-peak signal will contain input-crossover distortion while a 3-V peak-to-peak signal centered at 1.5 V will not contain input-crossover distortion as it avoids the crossover point. Another way to avoid large signal distortion is to use a gain of −1 circuit which avoids any voltage excursions at the input terminals of the amplifier. In that circuit, the common-mode DC voltage can be set at a level away from the V_OS cross-over point. For small signals, this transition in V_OS shows up as a V_CM dependent spurious signal in series with the input signal and can effectively degrade small signal parameters such as gain and common-mode rejection ratio. To resolve this problem, the small signal should be placed such that it avoids the V_OS crossover point. In addition to the rail-to-rail performance, the output stage can provide enough output current to drive 600-Ω loads. Because of the high-current capability, take care not to exceed the 150°C maximum junction temperature specification.

7.4.2 Input Bias Current Consideration

The LMV93x-Q1-N family has a complementary bipolar input stage. The typical input bias current (I_B) is 15 nA. The input bias current can develop a significant offset voltage. This offset is primarily due to I_B flowing through the negative feedback resistor, R_F. For example, if I_B is 50 nA and R_F is 100 kΩ, then an offset voltage of 5 mV will develop (V_OS = I_B x R_F). Using a compensation resistor (R_C), as shown in Figure 30, cancels this effect. But the input offset current (I_OS) will still contribute to an offset voltage in the same manner.

Figure 30. Canceling the Offset Voltage due to Input Bias Current