WPI’s Low-Noise Amplifiers Outperform Cheap Imitations AMPLIFIERS, ELECTROMETERS A n amplifier, in simplest terms, is an electronic device that magnifies an input signal. However, the way an amplifier is designed to handle noise and bandwidth limitations greatly affects the quality and sustainability of the final output signal. determined by the maximum voltage of the power supply. If the amplitude of the output signal is too large for the output range, part of the signal is cut off (clipped). • Rail – The upper or lower limit of the amplifier range is called a rail. Signals that exceed the rail cannot be faithfully reproduced. • DC Offset – DC offsets can appear in biological preparations. This offset is the amount the output signal is displaced away from a zero reference point, and it is usually a result of the potential difference at the electrode’s tip. reproduced. This is called “hitting the rail.” In our example, a 1.0μV input signal at an ×106 gain would generate a 1.0V output signal. Since the power supply is rated up to +5.0V, this output signal is clearly visible. If the input signal in this example is greater than 5.0μV, the output signal would be greater than +5.0V. Since 5.0V is the top of the range that the power supply is capable of producing, the output signal hits the upper rail and gets cut off. This amplifier will give a +5.0V DC output signal for all input signals greater than or equal to 5.0μV. In this instance, a smaller gain factor should be used to bring the output signal back into the dynamic output range of the amplifier. Noise Limits Amplifier Useability All electronic devices produce their own internal electronic noise, an unavoidable signal that can mask the output signal. For example, if the input signal is 2mV and the noise is 1mV, the signal to noise ratio is two to one (2:1), and the output signal would be undetectable. In this case, it is nearly impossible to discern which part of the output is generated by noise and which part is the desired signal. (Fig. 1) Signal vs. Noise Defining terms To knowledgeably discuss amplifiers, let’s define a few terms. • Gain – The gain is the multiplier defining how much the amplitude of an input signal is increased. A signal with an ×1 gain is not amplified. An ×10 gain produces an output signal ten times greater than the input signal. • Noise – Any unwanted signal fluctuations are called noise. While noise can also result from external sources, for the purpose of this discussion, we are primarily concerned with the noise resulting from the inner workings of the electronic device, our amplifier. This intrinsic noise is called shot (or schott) noise. • Signal to Noise Ratio (SNR) – The ratio of the output signal to the noise of the amplifier is called the signal to noise ratio. The smaller the shot noise signal in an amplifier in comparison with the output signal, the easier the desired signal is to discriminate. When engineering an amplifier, the SNR may be improved by boosting the first stage gain to yield a larger output signal or by using quality components to minimize the shot noise level of the amplifier. • Output Range – The output range determines the maximum output signal that can be generated with the amplifier. It is How does an amplifier work? Power Supply Rails Limits the Range In a perfect world an input signal can be infinitely multiplied by the gain factor to determine the output signal. For example: Input Signal 2mV 2mV 2mV 2mV 2mV Gain Output Signal ×1 2mV ×2 4mV ×10 20mV ×100 200mV ×10,000 20V In the real world, however, the power supply rails limit the possible output range of the amplifier. For example, a bio-amplifier could have a range of ±5.0V. In order for the output signal to be faithfully reproduced, the input signal times the gain factor must fall within the voltage window set by the power rails. Otherwise, the output signal will go off scale, and the input signal will not be faithfully Range { +5.0V Upper Rail Signal Noise -5.0V Lower Rail Fig. 1–The higher the signal to noise ratio, the more discernable the desired signal. 72 ORDER TOLL-FREE: 866-606-1974 (U.S. only) World Precision Instruments • Tel: 941-371-1003 • Fax: 941-377-5428 • E-mail: sales@wpiinc.com • Internet: www.wpiinc.com