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Current Feedback Amplifier Basics

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The basic current feedback amplifier topology is shown in Figure 1-14 below. Notice that within the model, a unity gain buffer connects the non-inverting input to the inverting input. In the ideal case, the output impedance of this buffer is zero (RO = 0), and the error signal is a small current, i, which flows into the inverting input. The error current, i, is mirrored into a high impedance, T(s), and the voltage developed across T(s) is equal to T(s)·i. (The quantity T(s) is generally referred to as the open-loop transimpedance gain.)
This voltage is then buffered, and is connected to the op amp output. If RO is assumed to be zero, it is easy to derive the expression for the closed-loop gain, VOUT/VIN, in terms of the R1-R2 feedback network and the open-loop transimpedance gain, T(s). The equation can also be derived quite easily for a finite RO, and Fig. 1-14 gives both expressions.
Current feedback (CFB) op amp topology
Figure 1-14: Current feedback (CFB) op amp topology
At this point it should be noted that current feedback op amps are often called transimpedance op amps, because the open-loop transfer function is in fact an impedance as described above. However, the term transimpedance amplifier is often applied to more general circuits such as current-to-voltage (I/V) converters, where either CFB or VFB op amps can be used. Therefore, some caution is warranted when the term transimpedance is encountered in a given application. On the other hand, the term current feedback op amp is rarely confused and is the preferred nomenclature when referring to op amp topology.
From this simple model, several important CFB op amp characteristics can be deduced.
  • Unlike VFB op amps, CFB op amps do not have balanced inputs. Instead, the noninverting input is high impedance, and the inverting input is low impedance.
  • The open-loop gain of CFB op amps is measured in units of Ω (transimpedance gain) rather than V/V as for VFB op amps.
  • For a fixed value feedback resistor R2, the closed-loop gain of a CFB can be varied by changing R1, without significantly affecting the closed-loop bandwidth. This can be seen by examining the simplified equation in Fig. 1-14. The denominator determines the overall frequency response; and if R2 is constant, then R1 of the numerator can be changed (thereby changing the gain) without affecting the denominator— hence the bandwidth remains relatively constant.
The CFB topology is primarily used where the ultimate in high speed and low distortion is required. The fundamental concept is based on the fact that in bipolar transistor circuits currents can be switched faster than voltages, all other things being equal. A more detailed discussion of CFB op amp AC characteristics can be found in Section 1-5.
Figure 1-15 below shows a simplified schematic of an early IC CFB op amp, the AD846— introduced by Analog Devices in 1988 (Reference 1: Wyn Palmer, Barry Hilton, "A 500V/μs 12 Bit Transimpedance Amplifier," ISSCC Digest, February 1987, pp. 176-177, 386). Notice that full advantage is taken of the complementary bipolar (CB) process which provides well matched high ft PNP and NPN transistors.
AD846 current feedback op amp (1988)
Figure 1-15: AD846 current feedback op amp (1988)
Transistors Q1-Q2 buffer the non-inverting input (pin 3) and drive the inverting input (pin 2). Q5-Q6 and Q7-Q8 act as current mirrors that drive the high impedance node. The CCOMP capacitor provides the dominant pole compensation; and Q9, Q10, Q11, and Q12 comprise the output buffer. In order to take full advantage of the CFB architecture, a high speed complementary bipolar (CB) IC process is required. With modern IC processes, this is readily achievable, allowing direct coupling in the signal path of the amplifier.
However, the basic concept of current feedback can be traced all the way back to early vacuum tube feedback circuitry, which used negative feedback to the input tube cathode. This use of the cathode for feedback would be analogous to the CFB op amp's low impedance (-) input, in Fig. 1-15.
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