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Transcript
Subject: High Speed Amplifiers Topic: Making High Speed Amplifiers Work (Understanding Performance Specifications) Part 1 Introduction Building a high speed amplifier circuit, with bandwidths in the mega-Hertz (MHz) range, requires an understanding of the amplifier specifications in great detail. As the bandwidths of the amplifier increase, the amplifiers specifications become even more necessary to understand due to the fact that many of the specifications interact with each other. It is important to note that system signal bandwidth is not the only important factor when designing a circuit; the actual bandwidth of the high speed amplifier itself (beyond specified signal bandwidth) can actually involve a combination of circuit parameters associated with the specifics of the circuit configuration. Remember, parasitic and nonlinear effects of the high speed amplifier beyond system signal bandwidth can cause excess system noise, overdrive, higher than expected distortion, and even DC offsets due to asymmetric slew rates. So a designer must take into account all the amplifiers specifications in regards to input signal frequencies as well as amplifier frequencies above (and below) the signal range when it comes to practically implementing the circuit. In the next few weeks, let’s focus on understanding amplifier specifications and how they affect performance and interact with each other. First, let’s start with an amplifiers unity gain bandwidth performance specification (UGBW). The UGBW is simply the amplifiers -3dB roll-off point (attenuation) of the frequency response for an amplifier configured for unity gain. The gain–bandwidth product (designated as GBWP, GBW, GBP or GB) for an amplifier is the product of the amplifier's bandwidth, and the gain at which the bandwidth is measured. For devices such as operational amplifiers that are designed to have a simple one-pole frequency response, the gain–bandwidth product is nearly independent of the gain at which it is measured (i.e. the gain-bandwidth product is constant); in that case, it will also be equal to the unity-gain bandwidth (UGBW) of the amplifier (the bandwidth at which the amplifier gain is set at a gain of 1). For an amplifier in which negative feedback reduces the gain to below the open-loop gain, the gain–bandwidth product of the closed-loop amplifier will be approximately equal to that of the gain-bandwidth product of the open-loop amplifier. Therefore, the parameter characterizing the frequency dependence of the operational amplifier gain is the finite gain–bandwidth product (GBWP). This quantity is commonly specified for operational amplifiers, and allows circuit designers to determine the maximum gain that can be extracted from the device for a given frequency (or bandwidth) and vice versa. It is also important to note, when adding LC circuits to the input and output of an amplifier, the gain can rise and the bandwidth may decrease, but the amplifier performance is generally bounded still by the gain–bandwidth product. For instance, if the UGBW of an operational amplifier is 1 MHz, it means that the gain of the device falls to unity at 1 MHz. Hence, when the device is wired for unity gain, it will work up to 1 MHz (UGBW = gain × bandwidth, therefore if BW = 1 MHz, then gain = 1) without excessively distorting the signal. The same device when wired for a gain of 10 will work only up to 100 kHz, in accordance with the UGBW product formula. Further, if the maximum frequency of operation is 1 Hz, then the maximum gain that can be extracted from the device is 1×106. Definition: UGBW: The -3dB roll-off point of the frequency response for an amplifier configured for unity gain, G = +1 (see below). By the way, many times, an amplifier operating at unity gain is at its least stable gain, and often times, some amplifiers are not actually unity gain stable. This is because at unity gain is where the amplifier is operating at its widest bandwidth configuration, and often times other higher frequencies poles decrease the amplifier phase margin. The following graphs represent various real amplifier responses at various gains. Also, pay attention to the various ways in which the bandwidths are specified: Definition: BWSS : Is the small signal bandwidth of a device at various gains with a small signal output ( <.5Vpp). Usually the bandwidth of most amplifiers, in this condition, for a given gain, remains constant at output voltages below the small signal maximum level (see below). BWLS or FPBW : Above small signal, the bandwidth usually decreases with increasing output swing. By the way, the bandwidth of a slew enhanced or CFB (current feedback amplifier) has a tendency to increase with increasing output swing before decreasing (see below). BW0.1dBSS or BW0.1dBls : The frequency at which the gain changes from its DC value by 0.1dB. This is a standard specification usually looked at by video engineers. They look for 0.1dB flatness through their video signal bandwidth. For example a high-definition application would require an amplifier with 0.1dB flatness to 30MHz. Another thing to think about in understanding the bandwidth specifications of an amplifier is to pay attention to the power supply voltages as well as the output power of the amplifier delivered to the load in a given application. Note that the bandwidth of the amplifier not only changes given a small signal or large signal voltage level (relative to the power supply rails) but be very careful to account for the current drive of the amplifier as well. Make sure to look over all the datasheet plots for more information on things like proper feedback resistor values, as well as the trade-offs between voltage output levels versus resistive loading (current drive levels). Remember, bandwidth IS the key specification that is used by the industry to “grade” a high performance amplifiers, but marketing also plays a key role in data sheet generation. Manufacturers usually specify the amplifier under the most optimal conditions, resulting in the best “looking” data sheet. So the designer must take into account each performance specification and how they relate to each other in a given application. Just be sure to read the fine print, and also look for performance plots to determine the amplifiers performance under your required operating conditions. For instance, a high speed amplifier driving a capacitive load can be very difficult to maintain amplifier stability (see the following): Driving Capacitive Loads Increased phase delay at the output due to capacitive loading can cause ringing, peaking in the frequency response, and possible unstable behavior. Use a series resistance, RS, between the amplifier and the load to help improve stability and settling performance. Refer to Figure 1. Figure 1 Addition of RS for Driving Capacitive Loads Table 1 provides the recommended RS for various capacitive loads. The recommended RS values result in <=0.5dB peaking in the frequency response. The Frequency Response vs. CL plot is also shown below Table 1 and illustrates the response of the CLC1605 Family. Table 1: Recommended RS vs. CL For a given load capacitance, adjust RS to optimize the tradeoff between settling time and bandwidth. In general, reducing RS will increase bandwidth at the expense of additional overshoot and ringing. When designing a high speed amplifier circuit, it is important to simply break down the system into the various functional blocks that make up the system and address each performance limiting factor. Depending on the overall system specification, such component versus signal bandwidths, and linearity versus noise requirements, these numbers will determine many of the required analog performance specifications of the system including simple layout geometries, amplifier bandwidths, slew rates, and required gains. The number one thing is to remember that every node in a circuit has some type of component connected to it and it is also both an input and an output in some way. Understanding the positive and adverse effects of this single concept will greatly enhance your ability to design the system. Kai ge from CADEKA (www.cadeka.com)