THS3001 Datasheet, PDF (22/32 Pages) Texas Instruments – 420-MHz HIGH-SPEED CURRENT-FEEDBACK AMPLIFIERS

THS3001, THS3002
420-MHz HIGH-SPEED CURRENT-FEEDBACK AMPLIFIERS
SLOS217A – JULY 1998 – REVISED JUNE 1999
APPLICATION INFORMATION
slew rate
The slew rate performance of a current-feedback amplifier, like the THS300x, is affected by many different
factors. Some of these factors are external to the device, such as amplifier configuration and PCB parasitics,
and others are internal to the device, such as available currents and node capacitance. Understanding some
of these factors should help the PCB designer arrive at a more optimum circuit with fewer problems.
Whether the THS300x is used in an inverting amplifier configuration or a noninverting configuration can impact
the output slew rate. As can be seen from the specification tables as well as some of the figures in this data sheet,
slew-rate performance in the inverting configuration is faster than in the noninverting configuration. This is
because in the inverting configuration the input terminals of the amplifier are at a virtual ground and do not
significantly change voltage as the input changes. Consequently, the time to charge any capacitance on these
input nodes is less than for the noninverting configuration, where the input nodes actually do change in voltage
an amount equal to the size of the input step. In addition, any PCB parasitic capacitance on the input nodes
degrades the slew rate further simply because there is more capacitance to charge. Also, if the supply voltage
(V_CC ) to the amplifier is reduced, slew rate decreases because there is less current available within the amplifier
to charge the capacitance on the input nodes as well as other internal nodes.
Internally, the THS300x has other factors that impact the slew rate. The amplifier’s behavior during the slew-rate
transition varies slightly depending upon the rise time of the input. This is because of the way the input stage
handles faster and faster input edges. Slew rates (as measured at the amplifier output) of less than about
1500 V/µs are processed by the input stage in a very linear fashion. Consequently, the output waveform
smoothly transitions between initial and final voltage levels. This is shown in Figure 51. For slew rates greater
than 1500 V/µs, additional slew-enhancing transistors present in the input stage begin to turn on to support
these faster signals. The result is an amplifier with extremely fast slew-rate capabilities. Figures 41 and 52 show
waveforms for these faster slew rates. The additional aberrations present in the output waveform with these
faster-slewing input signals are due to the brief saturation of the internal current mirrors. This phenomenon,
which typically lasts less than 20 ns, is considered normal operation and is not detrimental to the device in any
way. If for any reason this type of response is not desired, then increasing the feedback resistor or slowing down
the input-signal slew rate reduces the effect.
SLEW RATE
4
SLEW RATE
4
2
2
0
0
10
–2
5
SR = 1500 V/µs
0
Gain = 5
VCC = ±15 V
–5
RL = 150 Ω
RF = 1 kΩ
tr/tf = 10 ns
–10
5
SR = 2400 V/µs
0
Gain = 5
VCC = ±15 V
RL = 150 Ω
–5
RF = 1 kΩ
tr/tf = 5 ns
–10
–15
0 20 40 60 80 100 120 140 160 180 200
t – Time – ns
Figure 51
–15
0 20 40 60 80 100 120 140 160 180 200
t – Time – ns
Figure 52
22
• POST OFFICE BOX 655303 DALLAS, TEXAS 75265