Keywords: current conveyor, current feedback, operational amplifier, ideal transistor, higher-bandwidth, CCI, RF Mixers, High-Frequency Precision Rectifiers, source follower
Related Parts |

APPLICATION NOTE 4198

Abstract: Offering higher bandwidth than its voltage-feedback counterpart, the second-generation current conveyor operational amplifier (CCII) can be used in RF mixers, high-frequency precision rectifiers, and medical applications such as electrical impedance tomography. The conventional operational amplifiers cannot be used in the high-frequency applications due to their limited gain-bandwidth product.

Current conveyors are used in high-frequency applications where the conventional operational amplifiers cannot be used, because the conventional designs are limited by their gain-bandwidth product. In theory, the current conveyor is only limited by the f

From Figure 1 it can be seen that the CCII conveyor can be modelled as an ideal transistor:

Y being the base/gate

X being the emitter/source

Z being the collector/drain

This type of circuit works well as a circuit with BJTs, as the transconductance and Early voltages of BJTs are much greater than that of CMOS devices. Therefore, current conveyors work well as source followers. Gain X/Y is close to 1; Z has a natural high-output impedance which cannot be mimicked by their CMOS counterparts.

Where g

A typical simulated gain with a TSMC 0.18µm with a load of 1kΩ gave a gain of 0.7. Compared to the ideal gain of 1, this represents a 30% loss in output gain.

Figure 2a can be implemented as shown below in

From Figure 2b it can be seen that output X is fed back to one of the long tail pairs of inputs (X'). The other input to the long tail pair is Y, as input Y changes the current through M1. M2 differs as M3, and M4 is a current mirror.

There is a current difference between M2 and M4. This imbalance is addressed by pulling current from, or to, the gate/source capacitance C

The current from M5/M6 is simply mirrored by M7/M8, giving the output Z(-) of the CCII+.

The output impedance of Z can be improved by adding in a cascode to M7/M8 if necessary. One must be aware that to mimic the current successfully, the output impedance of X must match that of Z, i.e., the same transistor types and confirmation must be used on M5/M6 as on M7/M8.

The gain of the CCII is simply:

From Figure 4, if all transistor dimensions are the same and if Yb' (the bias point from Figure 3) is taken, then a current 2i is generated from M10 and M11. This is mirrored by M9 to give a current of 2i through M13. M12 provides a current of i and gives a current of -i through Z(+), thus giving a true CCII- output. There is a problem with this approach: the Z(+) now has an -i DC term instead of a +i term. Consequently, a 2i DC term needs to be added to the output of Z(+) to compensate for -i.

From Figure 5, transistors M14 and M15 provide the appropriate current to compensate for the DC current taken by M13. (Note that M14 and M15 must match M12). Make the current through R3 equal to i(DC) - i'. Remember that R3 and R2 must match; any mismatch in their values will mean that their output DC values will differ.

The only problem with this design is that another resistor (R4) is needed, and R4 must again match R2 and R3.

These results were improved by using a cascode device to replace M5/M6 and M7/M8, which gave a bandwidth of 900MHz and an improved gain of 0.993. The PSRR was also improved at 51dB.

¹K.C. Smith and A. Sedra, 'The Current-Conveyor — A New Circuit Building Block,'

²C. Toumazou, John Lidgey & Alison Payne, 'Practical Integrated Current-Conveyors, Current Mode Circuits Techniques in Analog High Frequency Design,' July 1996, Chapter 5.2, pp. 69–80.

³K.C. Smith and A. Sedra, 'A Second Generation Current-Conveyor and its Applications,'

This application note is based on an article published in