Column: Circuit drill
Composite amplifier phase shift at different input signal frequencies
By Dr. Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions
drawbacks. They are used when a single amplifier can’t meet all requirements, such as high speed, high voltage, high output, high DC precision, low noise and low distortion. The circuit shown in Figure 1 shows
A
a simple composite amplifier with two op-amps (LM471), designed to operate as a high-speed unity-gain inverter. When designing an electronic
circuit, it is important to ensure that it delivers the intended output without adding any unwanted effects that could hinder its reliability or functionality, in all circumstances. For example, I wondered how this circuit’s phase shifts behaves. Is there considerable phase shift between the input and the output signals? If so, what’s the magnitude? And, will that change with change in input frequency? Here we will examine this circuit for
delivering output signals with the same phase shift as that of the input signal.
Experiment setup The circuit shown in Figure 1 was chosen for the experiment because of
10 March 2024
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composite amplifier combines two or more amplifiers, maximising their benefits and minimising their
its simplicity. However, there are many alternatives, all operating on the same principle. During the experiment, the input
signal’s frequency was varied, checking the corresponding output’s phase shift. Input frequency ranged from 1kHz to 1MHz, in 1kHz increments. The upper limit of 1MHz was chosen to ensure stable performance of our op-amp (LM471), which is specifically designed to handle frequencies to 1.5MHz. All other component values were kept fixed throughout the experiment. The output signal’s phase shift was
measured at each 1kHz step; see Figure 3. It is important to note that the phase shift values, which indicate the time delay between the input and output signals, were measured in degrees. This measurement was obtained by dividing the monitored time delay by the signal wavelength at each signal frequency, and then multiplying it by 360o equation is as follows:
. The
Phase Shift (in degrees) = (T2-T1) * 360 / WL, where T2 represents the time point of the output signal and T1 represents the time point of the input signal; see Figure 2.
From Figure 3 it is clear that the
circuit shows a significant phase shift between the input and output signals.
This phase shift covers a wide range – from -200o -350o
(lagging) at input signal
frequencies of 80kHz and 1MHz, respectively. It is important to note that
this phase shift is a crucial characteristic of the circuit's behaviour and plays a vital role in its overall functionality and performance.
Results The experiment shows that the output signal consistently lags behind the phase of the input signal, with phase shift magnitude that changes with the input signal’s frequency. For instance, at an input frequency of 80kHz, the phase shift is approximately -200o
(lagging),
whilst at 1MHz, the phase shift increases to about -350o
(lagging). This discovery is important for
circuit designers who consider using this circuit. They should carefully assess the potential impact of the induced phase shift on the downstream connected circuit. If the phase shift is deemed significant and problematic in such circumstances, it is recommended to explore alternative circuits that offer a lower degree of induced phase shift.
(lagging) to
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