Electronics Guide

Bandwidth Advantages

The bandwidth advantage of current-mode circuits stems from the low-impedance nodes inherent in current-mode signal transfer. When a current signal is conveyed through a low-impedance node, the voltage across any parasitic capacitance remains small, eliminating the RC time constant that would otherwise limit bandwidth. This principle enables current-mode circuits to achieve bandwidths approaching the intrinsic transistor cutoff frequency, far exceeding what equivalent voltage-mode circuits can attain.

Consider a simple current transfer between two points. In a voltage-mode implementation, a buffer must charge the output node capacitance through its finite output impedance. In a current-mode implementation using a current conveyor, the input node is held at virtual ground by the low-impedance input port, while the output current is delivered through a high-impedance current source that is inherently fast regardless of load capacitance. The bandwidth is then limited only by internal transistor speeds rather than external node capacitances.

Dynamic Range Considerations

Current-mode circuits often achieve superior dynamic range compared to voltage-mode equivalents, particularly when combined with logarithmic compression techniques. The exponential relationship between base-emitter voltage and collector current in bipolar transistors provides natural logarithmic compression that can handle signals spanning many decades. This compression occurs at the fundamental device level, enabling dynamic ranges exceeding 100 dB in practical circuits.

The noise performance of current-mode circuits depends critically on the current levels and transistor configurations employed. While shot noise in current-mode circuits scales with signal current, the inherent signal compression in many current-mode topologies allows the noise floor to compress along with small signals, maintaining signal-to-noise ratio across a wide range of input levels.

First-Generation Current Conveyor (CCI)

The first-generation current conveyor (CCI) has three terminals: X, Y, and Z. The X terminal presents a low input impedance, the Y terminal presents a high input impedance, and the Z terminal provides a current output. The defining relationships are that the voltage at X follows the voltage at Y (VX = VY), and the current into X is conveyed to the Z terminal (IZ = IX). The Y terminal draws negligible current.

While the CCI provided the conceptual foundation for current-mode circuit design, its practical utility was limited by the bidirectional current relationship at the X terminal, which made it difficult to implement certain functions. The second-generation current conveyor addressed these limitations.

Second-Generation Current Conveyor (CCII)

The second-generation current conveyor (CCII) modified the terminal relationships to provide more practical functionality. In the CCII, the X terminal voltage still follows the Y terminal (VX = VY), but the current relationship is unidirectional: the current flowing into the X terminal is conveyed to the Z terminal, while the Y terminal presents infinite impedance and draws no current.

The CCII comes in two polarities: CCII+ where IZ = +IX, and CCII- where IZ = -IX. This flexibility enables implementation of a wide variety of circuit functions. A CCII can be constructed from a pair of complementary current mirrors and a voltage buffer, or from operational amplifiers configured appropriately. Modern integrated implementations achieve bandwidths exceeding several hundred megahertz.

The CCII enables remarkably simple implementations of many common circuit functions. A current amplifier requires only a CCII with appropriate resistor ratios. Voltage amplifiers, integrators, differentiators, and filters can all be constructed using CCIIs with fewer components than equivalent op-amp implementations. The inherent bandwidth advantage of current-mode operation typically results in superior high-frequency performance.

Third-Generation and Beyond

Subsequent generations of current conveyors have added features such as controllable current gain, multiple output terminals, and differential inputs. The controlled current conveyor (CCCII) adds a control input that adjusts the transconductance, enabling voltage-controlled amplification. The differential voltage current conveyor (DVCC) accepts differential voltage inputs, combining the benefits of current-mode processing with differential signal handling.

These advanced current conveyors enable more sophisticated signal processing functions while maintaining the bandwidth and dynamic range advantages of current-mode operation. Many modern analog integrated circuits incorporate current conveyor principles in their internal architectures, even when presenting traditional voltage-mode interfaces to the user.

Operating Principles

The current feedback amplifier topology employs a low-impedance inverting input that senses current rather than voltage. The non-inverting input presents high impedance like a conventional op-amp, but the inverting input connects to the emitter of a complementary transistor pair, presenting an impedance of only a few tens of ohms. When the inverting input sources or sinks current through the feedback network, this current is amplified by a transimpedance stage to produce the output voltage.

The gain equation for a current feedback amplifier in the inverting configuration is approximately -RF/RG, where RF is the feedback resistor and RG is the gain-setting resistor. Critically, the bandwidth depends primarily on RF alone, not on the closed-loop gain. This results from the current-sensing nature of the inverting input: higher gains require larger input currents for the same output, and these larger currents are delivered by the low-impedance input without the delay associated with charging input capacitance.

Design Considerations

While current feedback amplifiers offer exceptional bandwidth performance, their design requires attention to several unique considerations. The feedback resistor value is critical: too small a value can cause instability, while too large a value reduces bandwidth. Manufacturers specify optimal feedback resistor values for different supply voltages and load conditions.

The low-impedance inverting input has implications for circuit design. Capacitance directly on the inverting input, whether from circuit traces or intentional compensation, can cause instability. Unlike voltage feedback amplifiers where input capacitance merely reduces bandwidth, input capacitance on a CFA inverting input can cause peaking or oscillation. Careful layout with minimal trace length to the inverting input is essential.

Current feedback amplifiers also exhibit different noise characteristics than voltage feedback types. The input current noise is typically higher due to the low-impedance input stage, making CFAs less suitable for high-impedance source applications. However, their superior slew rate and bandwidth make them excellent choices for driving cables, ADC inputs, and other applications requiring fast settling.

The Translinear Principle

The translinear principle states that in any closed loop of bipolar transistor base-emitter junctions, arranged with an equal number of junctions in each direction around the loop, the product of current densities in the clockwise devices equals the product of current densities in the counterclockwise devices. For transistors with equal areas, this simplifies to the product of currents being equal.

Mathematically, if transistors Q1 and Q2 are oriented clockwise in a loop while Q3 and Q4 are oriented counterclockwise, then IC1 * IC2 = IC3 * IC4. This relationship holds because the sum of base-emitter voltages around the loop must be zero, and the exponential VBE-IC relationship transforms this voltage constraint into a current product constraint.

The elegance of the translinear principle is that it depends only on the exponential characteristic of the transistors, not on absolute values of parameters like saturation current. Temperature variations affect all transistors equally, providing inherent temperature compensation. Process variations similarly cancel, enabling integrated circuits with exceptional accuracy.

Translinear Multipliers

The four-quadrant analog multiplier represents perhaps the most important application of translinear principles. The Gilbert cell multiplier, widely used in communications systems for mixing and modulation, implements multiplication using two differential pairs arranged in a translinear configuration. The output current is proportional to the product of the two input voltages, with all four quadrant operation (both inputs and output can be positive or negative).

Gilbert cell multipliers achieve multiplication accuracies better than 1% over a useful dynamic range. The bandwidth can extend to several hundred megahertz in modern processes, making these circuits essential for RF applications. Variations of the basic Gilbert cell implement variable-gain amplifiers, mixers, modulators, and phase detectors, all exploiting the fundamental translinear multiplication operation.

Geometric Mean and RMS Computation

Translinear circuits excel at computing mathematical functions that would require complex algorithms in digital systems. The geometric mean of two currents emerges naturally from a simple translinear loop: if IC1 * IC2 = IC3 * IC4 and we set IC3 = IC4 = IOUT, then IOUT equals the square root of IC1 * IC2, which is the geometric mean.

RMS (root mean square) computation extends this principle. By using a translinear squaring circuit followed by averaging and a square-root circuit, true RMS values can be computed for arbitrary waveforms. These RMS-to-DC converters are essential for accurate AC measurements and are found in precision multimeters and power measurement instruments. The translinear implementation provides accuracy independent of waveform shape, correctly measuring the true power content of complex signals.

Companding Principle

Log-domain filters employ compression at the input and expansion at the output, a technique borrowed from telecommunications called companding. The input current is logarithmically compressed as it enters the filter, processed in the compressed domain, and then exponentially expanded to recover the output. Because the compression and expansion are precisely matched through the translinear loop principle, the overall transfer function remains linear despite the internal nonlinear representation.

The advantage of this approach is that internal signal swings are dramatically reduced. A signal with 60 dB of dynamic range (1000:1 current ratio) compresses to only 60 mV of internal voltage swing when represented logarithmically by transistor base-emitter voltages. This small internal swing enables operation at frequencies and dynamic ranges that would be impossible with conventional linear signal representation.

Log-Domain Integrators and Filters

The fundamental building block of log-domain filtering is the log-domain integrator. Unlike conventional integrators that use capacitors to integrate current into voltage, log-domain integrators use the exponential transistor characteristic to implement integration in the compressed domain. The time constant of a log-domain integrator is set by a bias current, enabling electronic tuning of filter characteristics.

Complete filters are constructed by interconnecting log-domain integrators according to standard filter synthesis techniques. State-variable, Gm-C, and other topologies all have log-domain equivalents that provide the same transfer functions with enhanced performance. Log-domain filters have demonstrated operation at frequencies exceeding 100 MHz with dynamic ranges over 60 dB, performance that would be extremely difficult to achieve with conventional approaches.

The electronically tunable nature of log-domain filters makes them attractive for applications requiring adaptive filtering or wide tuning ranges. By varying bias currents, the cutoff frequency can be adjusted over several decades without changing component values, enabling applications like automatic gain control loops and frequency-agile communication systems.

Class AB Log-Domain Circuits

Early log-domain circuits operated in class A, with bias currents that had to exceed the maximum signal current. This limited efficiency and created challenges for large-signal operation. Class AB log-domain circuits address these limitations by using push-pull current structures that can handle signals larger than the quiescent bias current.

In class AB operation, two complementary signal paths handle positive and negative excursions of the signal current. Each path operates in class A individually, but the composite structure provides class AB operation where the total current can greatly exceed the quiescent current. This approach dramatically improves power efficiency and enables log-domain processing of large signals without excessive power consumption.

Gm-C Filters

Transconductance-capacitor (Gm-C) filters represent a bridge between voltage and current modes. Transconductance amplifiers convert input voltage to output current, and when combined with capacitors, implement integrators and complete filters. The time constant of a Gm-C integrator is C/Gm, and by varying the transconductance through bias current adjustment, the filter can be electronically tuned.

Modern Gm-C filters achieve operating frequencies from kilohertz to hundreds of megahertz, with applications in disk drive read channels, communications systems, and sensor interfaces. The transconductors can be implemented using differential pairs, current conveyors, or more sophisticated structures optimized for linearity. Fully integrated implementations benefit from the ratiometric nature of capacitor matching in integrated circuits.

Current-Mirror-Based Filters

Filters can be constructed entirely from current mirrors and capacitors, avoiding the need for resistors entirely. These current-mirror-based filters are particularly attractive for integrated circuit implementation where accurate resistors are difficult to fabricate. The frequency response depends on current ratios and capacitor values, both of which can be accurately controlled in integrated processes.

Basic building blocks include current amplifiers, current integrators, and current differentiators. A current integrator is formed when a current mirror drives a grounded capacitor: the output current is the derivative of the capacitor voltage, which is the integral of the input current. By interconnecting these blocks, complete filter transfer functions can be realized.

Basic Mirror Topologies

The simple two-transistor current mirror copies the input current to the output with an accuracy limited by transistor matching and the Early effect. For bipolar implementations, matching of base-emitter voltages sets the DC accuracy, while finite beta reduces accuracy by diverting a fraction of the input current into the mirror transistor bases. For MOS implementations, threshold voltage matching and channel-length modulation are the primary error sources.

Wilson and improved Wilson mirrors add a third transistor that bootstraps the mirror input, dramatically improving output impedance and reducing errors from output voltage variations. Cascode mirrors achieve similar improvements by shielding the mirror transistor from output voltage changes. These enhanced topologies enable current mirrors with output impedances exceeding tens of megohms and accuracy better than 0.1%.

Wide-Swing and High-Speed Mirrors

High-performance current-mode systems require mirrors that combine high output impedance with wide output voltage compliance and high bandwidth. Wide-swing cascode mirrors achieve high impedance while allowing output swings within a few hundred millivolts of the supply rails. Regulated cascode mirrors use local feedback to maintain the mirror transistor drain voltage, achieving extremely high output impedance without sacrificing compliance range.

Bandwidth optimization in current mirrors involves careful attention to the capacitances at internal nodes. The dominant pole typically occurs at the input node, where the input current must charge the gate or base capacitances of the mirror transistors. High-speed mirrors often incorporate local feedback or compensation techniques to extend bandwidth while maintaining stability.

Current Memory Cells

Current memory cells, also called current copiers with memory, can sample and hold a current value for later use. The basic concept stores the controlling voltage of a current source on a capacitor, then disconnects the input while maintaining the output current. These cells are essential for sample-and-hold functions in current-mode systems and form the basis of current-mode analog-to-digital and digital-to-analog converters.

First-generation current memory cells suffered from errors due to clock feedthrough and incomplete settling. Second-generation cells use two-phase operation where the memory capacitor is first charged to the correct value, then isolated before the output transistor is connected. This regulated cascode approach dramatically improves accuracy, enabling current memory cells with precision suitable for 10-bit and higher resolution data conversion.

Current-Mode DACs

Many digital-to-analog converters use current-mode architectures, even when presenting voltage outputs. Binary-weighted current sources are switched according to the digital input, and the sum of selected currents is converted to voltage through a transimpedance amplifier or resistor. This architecture is particularly well-suited to segmented implementations where the most significant bits use thermometer coding for improved linearity.

The current steering DAC represents a highly successful current-mode topology. Pairs of current sources are steered to either the output or a dummy load based on the digital input. Because the current sources run continuously and are merely redirected, switching glitches are minimized and settling is fast. Current steering DACs with sampling rates exceeding 10 Gsps have been demonstrated, making them essential for high-speed communications and signal generation.

Current-Mode ADCs

Analog-to-digital converters can also benefit from current-mode techniques. Algorithmic ADCs that use current copiers to store and process samples can achieve high resolution with compact implementations. The successive approximation algorithm can be implemented using current DACs and comparators, with the current-mode DAC providing fast settling for high conversion rates.

Pipeline ADCs often use current-mode interstage amplifiers to take advantage of the speed benefits. The residue from each stage is converted to current, amplified, and passed to the subsequent stage. Current-mode implementations can achieve the high interstage gain required for pipeline operation at speeds difficult to attain with voltage-mode approaches.

Push-Pull Current Structures

Class AB current processing typically employs complementary NMOS and PMOS (or NPN and PNP) transistor pairs that share the signal current. For small signals near zero, both transistors conduct approximately equal quiescent currents. As the signal increases positively, the sourcing transistor conducts more while the sinking transistor conducts less, with the signal current being the difference. The reverse occurs for negative signals.

The key challenge in class AB current-mode design is maintaining the relationship between the two transistor currents such that their difference accurately represents the signal. Various techniques have been developed, including translinear loops that constrain the product of complementary currents to a constant value, and local feedback structures that sense and correct deviations from the desired operating point.

Class AB Current Mirrors and Conveyors

Class AB current mirrors extend the operating range of basic current mirrors to handle bidirectional currents larger than the quiescent bias. The minimum selector, proposed by Seevinck, uses the minimum of two transistor gate voltages to control the bias, ensuring that at least one transistor always conducts regardless of signal polarity. More sophisticated structures achieve smoother transitions between sourcing and sinking modes.

Class AB current conveyors combine the versatility of the current conveyor architecture with the large-signal handling capability of class AB operation. These devices are essential for practical current-mode systems that must handle wide dynamic range signals without excessive power consumption. Modern class AB current conveyors achieve bandwidths of hundreds of megahertz while handling signal currents many times larger than the quiescent current.

Communications Systems

RF and wireless communications extensively use current-mode circuits for their superior high-frequency performance. Gilbert cell mixers implement frequency conversion in receivers and transmitters. Current-mode variable-gain amplifiers provide automatic gain control with wide tuning range. Current-steering DACs generate high-frequency signals for direct digital synthesis and communications transmitters.

Sensor Interfaces

Many sensors naturally produce current outputs, making current-mode processing a natural fit. Photodiodes generate currents proportional to incident light. Current-output temperature sensors and magnetic field sensors benefit from current-mode signal conditioning that preserves their inherent accuracy. Translinear circuits enable precise ratiometric measurements that are insensitive to absolute scale factors.

Neural and Biological Systems

Current-mode circuits have found application in neuromorphic engineering, where the goal is to emulate biological neural processing. The logarithmic response of bipolar transistors mimics the logarithmic response of biological photoreceptors. Current-mode computation naturally implements the summation of synaptic inputs in neural models. Low-power current-mode circuits enable large-scale neural network implementations within practical power budgets.