Imaging equipment such as X-ray machines, florescence-reaction detectors, and nuclear-medicine sensors generate extremely low current output signals, in the nano-amp range, which must be amplified and measured before that equipment can produce useful images. Transconductance amplifiers are one way for doing so.

The amplifier converts an input current to an output voltage. Typical input currents for modern transconductance amplifiers range from a couple of pico-amps to tens of nano-amps. The operational amplifiers used to boost such small signals have extremely high input impedance and are often referred to as electrometer grade.

Transconductance amplifiers are made possible by the availability of many electrometer-grade operational amplifiers from Analog Devices and Texas Instruments. Favorites, such as Analog Devices' AD549 or Texas Instruments' OPA128, have extremely high input impedance and low input bias offset current. Typical input bias currents for these devices are less than a few hundred femto-amps. Such a low input bias makes possible meaningful measurements of pico-amp input currents.

The accompanying circuit topology shows a simplified transconductance amplifier. Low current is injected directly into the minus input of an electrometer grade operational amplifier. Resistance R is typically between 10 and 1,000 M-ohms. Working with low current and high-resistance components requires careful layout and trace routing on the circuit board. When the design demands high performance at the lowest currents, it's mandatory to ground-guard the minus-input node, lift the minus-input node off the circuit board, and shielding the entire circuit.

Experience teaches that amplifiers with input-current requirements in the nano-amp range can often achieve a relatively high frequency response of some tens of kilohertz. Such amplifiers tend to be stable and less susceptible to stray signal pickup than other designs.

Amplifiers for extremely low current, in the pico-amp range, represent a greater challenge. Such low-current amplifiers often require that circuitry perform automatic zero-offset corrections to make up for sensor-component drift. These amplifiers also require careful circuit board layout and shielding. A technique we find useful makes the entire secondary side of the circuit board the shield bottom and a metal can shields the top. This is effective with low magnetic-field radiation. Where external interference is high, consider using mu-metal shielding to reduce the effects of external noise sources on amplifier performance.

In addition, the first power up of an extremely low-current input amplifier can be confusing to novice designers. Amplifier output that is pegged to the positive or negative rail is a common occurrence. Getting the amplifier under control so one can believe its working requires patients and a light touch. A good supply of shielded, precision, and low-current sources can greatly reduce debug and checkout time.