FEATURE COVER STORY
TRANSIMPEDANCE AMP FOR HIGH SPEED HIGH DYNAMIC RANGE PHOTODIODE CIRCUITS
Brian Black, product marketing manager and Glen Brisebois, senior applications engineer at Linear Technology Corporation(now part of Analog Devices Inc.) investigate how the latest amplifiers are enabling high performance in wide range photodiode circuits for chromatography and medical imaging
P
hotodiodes transform light into a current or voltage, which can then be
used in electronic circuits in applications ranging from solar cells to optical data networks, from precision instruments to chromatography to medical imaging. What all these applications have in common is the need for circuitry to buffer and scale the photodiode output. For applications requiring high speed and high dynamic range, transimpedance amplifier (TIA) circuits like the one shown in Figure 1 are often used. In this figure, the feedback capacitance is shown as a parasitic capacitance. In many applications this will be a deliberately placed capacitor to ensure stability. This circuit has the photodiode in
“photoconductive mode” with a bias voltage applied to the cathode. The virtual connection between the two op amp inputs holds the anode at ground, thus applying a constant reverse bias voltage across the photodiode. A photodiode can be thought of as a
current source (proportional to light intensity), a capacitor, a large resistor, and a so-called dark current (the small current when no photons are present) all connected in parallel. The larger the bias voltage across the diode, the smaller the photodiode capacitance tends to become. While this is good for speed, it is limited practically by the capability of a photodiode to withstand large reverse voltages. The current generated by the
photodiode (IPD ) is amplified by the TIA
circuit and converted to an output voltage through the transimpedance gain resistor (also referred to here as the feedback resistor, or RF current flows through RF
). Ideally this (i.e., IFB
= IPD ),
but in practice the amplifier introduces an error current in the form of op amp input bias current. This bias current results in an error voltage at the output and limits dynamic range. The larger the gain resistor, the greater this effect. It is important to select an amplifier with
12 OCTOBER 2017 | ELECTRONICS
Figure 1: Transimpedance amplifier
sufficiently low bias current (as well as input offset voltage and input offset voltage drift) to achieve the required dynamic range and overall accuracy. One other consideration is the effect of
op amp input current variation over temperature. Op amps with bipolar input stages have fairly constant input current, but this current is so high even at room temperature (nA or even µA) that unbuffered bipolar amplifiers are not suitable for many high transimpedance gain applications. For this reason, op amps with a FET input stage are often preferred over bipolar amplifiers because they have inherently lower input current – often in the single digit picoampere range or even lower at room temperature. But input ESD protection diodes leak as they get hot, causing the input current to rise exponentially with temperature. It is not unusual for an op amp with picoampere bias current at room temperature to have nanoampere input current at 125°C. One alternative is to use a discrete FET to buffer the
/ ELECTRONICS
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56
