14.8. Output Circuit of the TS271 Opamp

The TS271 operational amplifier is designed to operate at very low current levels. This would include the current of the source follower of the output stage. In this case, the output resistance of an amplifier with a simple source-follower stage for an output stage, such as in Fig. 14.7, would be unacceptably high. In the TS271 operational amplifier, the 9-transistor circuit of Fig. 14.8 replaces the simple source-follower stage consisting of M₆ and M₁₃ of Fig. 14.7. Note that the circuit of Fig. 14.8 uses different transistor designators. In Fig. 14.7, M₆ and M₁₃ are equivalent to M₁ and M_n1 of Fig. 14.8.

The circuit of Fig. 14.8 has an additional source-follower stage consisting of M₂ and M_n2. The output from the common-source stage (drain of M₃ of Fig. 14.7) is applied to the gates of both source-follower stages. The sources (outputs) of M₁ and M₂ are connected to the inputs of the differential-stage gates of M₃ and M₄, respectively. There is no load at the source of M₁ such that the gain of the source follower can be considered unity. Thus, V_g3 ≈ V_g2 = V_g1 (incremental voltages). By contrast, the source of M₂ is connected to R_L such that, in general, V_o = V_s2 < V_g2.

From another perspective, suppose V_g2 = 1V, that is the source-follower gates are raised by 1V above the bias level. With a load, R_L, the current increase will be greater for M₂ compared to M₁. This will cause the source voltage of M₂ to be slightly lower (than that of M₁) such that the input to the differential amplifier is nonzero and the output voltage (drain of M₃) will drop. The result is a reduction of the gate voltage of M_n2 and thus a reduction of the drain current in M_n2. The incremental drain current of M_n2 provides most of the current through the load; the load current is dominated by the change in the current of M_n2 instead of M₂.

Fig. 14.9 is a reasonable equivalent, incremental, circuit. The differential stage (M₃, minus, M₄, plus, and the drain of M₃ as the output) is represented by an opamp symbol.

Since the source follower of M₁ has a transfer function of nearly unity, incremental V_g2(= V_g1) is moved directly to gate of M₃ (minus input of the differential amplifier) as in Fig. 14.9. Thus

Equation 14.39

where a_vda is the gain of the opamp and the output of the opamp is V_gsn2. The amplifier (output stage) output voltage is therefore

Equation 14.40

where we now assume that g_m2 = g_mn2. It follows that the relation between V_gs2 and V_o is

Equation 14.41

Combining this with

Equation 14.42

gives the transfer function, input to output, as

Equation 14.43

where

Equation 14.44

The output resistance of the circuit of Fig. 14.8 is 1/G_m. The output resistance of the circuit of Fig. 14.8 is thus reduced by a factor of about 1/a_vda compared to the circuit of Fig. 14.7 for the same g_m of the source follower transistors. From (14.18), the gain of the differential-amplifier stage is

This would typically be on the order of 100. Thus a significant benefit over the simple source-follower stage output is realized with the circuit of Fig. 14.8. It is also significant that there is no body effect associated with M_n2.