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The search for the ideal transistor

30 Mar 2015  | Sergio Franco

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We accomplish this task by means of two current mirrors with the outputs tied together, as depicted in figure 3 for the case of Wilson-type current mirrors. The Q5-Q6-Q7 mirror replicates iC1 and sources it into the output node, whereas the Q8-Q9-Q10 mirror replicates iC2 and sinks it out of the output node. For vB = 0, the exponentials of Equation (3) cancel each other out to give iC = 0. For vB > 0, iC1 prevails to give iC > 0, and for vB < 0, iC2 prevails to give iC < 0. Clearly, the circuit allows for full four-quadrant operation. Moreover, it exhibits high input resistance thanks to the Darlington function provided by the Q3 and Q4 emitter followers, and high output resistance because of the Wilson mirrors.

Well, this pseudo-ideal BJT has been around for quite some time. Variously called a transductor, a macro transistor, a diamond transistor, and a Current Conveyor II+ [1], it is also available in IC form as the OPA861 [2]. Figure 4 shows how much simpler the amplifier of figure 1 would be if implemented with a transductor. Note that the amplifier of figure 1 provides signal inversion, whereas that of figure 3 is of the non-inverting type.

Figure 4: Implementing a common-emitter amplifier with a transductor.

The current-feedback amplifier (CFA)
As we know, the range of applications of an amplifier can be expanded dramatically through the use of negative feedback, and a transductor-based amplifier is no exception. Since the transductor exhibits high output resistance, we need to use an output buffer to prevent loading by the feedback network. This leads us to the circuit of figure 5, where the output buffer, consisting of Q11 through Q14, is similar to the input buffer Q1 through Q4. This circuit too has been around for quite some time [3]. Called a current-feedback amplifier (CFA), it replaces the conventional op amp in certain high-speed applications. Unlike the ordinary BJT, which we configure for negative-feedback operation by connecting the feedback network between the collector and the base, the non-inverting nature of the transductor requires that the feedback network be connected between its (buffered) collector and the emitter, that is, between the vO and vN nodes of figure 5.

To investigate feedback operation, refer to the simplified equivalent of figure 6a, showing explicitly the net impedance zc presented by the C node towards ground (for reasons that will become clear shortly, the C node is also called the gain node). To a first approximation, zc can be modelled with a resistance Rc in parallel with a capacitance Cc, so expanding gives

Typically, Rc in the range of 105~106 Ω and Cc is in the pF range. Now, any current imbalance In created by the external network will get replicated by the Wilson mirrors at the C node to give

Vo = zcIn           (5)

Figure 5: Using an output buffer to turn a transductor into a current-feedback amplifier (CFA).

Turning next to the typical feedback interconnection of figure 6b, we sum currents into the Vn node to get

Letting Vn = Vp = Vi, solving for In, and inserting into Equation (5) gives the closed-loop voltage gain

Figure 6: (a) Simplified equivalent of the CFA. (b) CFA symbol and interconnection for negative-feedback operation as a non-inverting amplifier.

In a well-designed circuit, R2 is on the order of 103 Ω, so with Rc in the range of 105~106 Ω, we can ignore the term R2/zc at dc and state that at low frequencies A tends to the familiar op amp expression

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