Download Subject: High Speed Amplifiers Topic: Making High Speed Amp

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)