H1115I Просмотр технического описания (PDF) - Intersil

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H1115I

225MHz, Low Power, Output Limiting, Closed Loop Buffer Amplifier

Intersil 

HFA1115

QP3

V+

QP4

QN2

R1 50kΩ

+IN

QP1

V-

ILIMIT

Z

+1

V+

QN1

QP2

QN5

QN6

QP6

200Ω VH

QN3

QN4 QP5

V-

V-IN

RG = 350Ω

RF = 350Ω

(INTERNAL) (INTERNAL)

-IN

VOUT

FIGURE 3. HFA1115 SIMPLIFIED VH LIMIT CIRCUITRY

This current is mirrored onto the high impedance node (Z) by

QX3-QX4, where it is converted to a voltage and fed to the

output via another unity gain buffer. If no limiting is utilized,

the high impedance node may swing within the limits defined

by QP4 and QN4. Note that when the output reaches its

quiescent value, the current flowing through -IN is reduced

to only that small current (-IBIAS) required to keep the output

at the final voltage.

Tracing the path from VH to Z illustrates the effect of the limit

voltage on the high impedance node. VH decreases by 2VBE

(QN6 and QP6) to set up the base voltage on QP5. QP5

begins to conduct whenever the high impedance node

reaches a voltage equal to QP5’s base voltage + 2VBE (QP5

and QN5). Thus, QP5 limits node Z whenever Z reaches VH.

R1 provides a pull-up network to ensure functionality with the

limit inputs floating. A similar description applies to the

symmetrical low limit circuitry controlled by VL.

When the output is limited, the negative input continues to

source a slewing current (ILimit) in an attempt to force the

output to the quiescent voltage defined by the input. QP5

must sink this current while limiting, because the -IN current

is always mirrored onto the high impedance node. The

limiting current is calculated as:

ILIMIT = (V-IN - VOUT LIMITED)/RF + V-IN/RG.

As an example, a unity gain circuit with VIN = 2V, and VH = 1V,

would have ILIMIT = (2V - 1V)/350Ω + 2V/∞ = 2.8mA (RG = ∞

because -IN is floated for unity gain applications). Note that ICC

increases by ILIMIT when the output is limited.

Limit Accuracy

The limited output voltage will not be exactly equal to the

voltage applied to VH or VL. Offset errors, mostly due to VBE

mismatches, necessitate a limit accuracy parameter which is

found in the device specifications. Limit accuracy is a

function of the limiting conditions. Referring again to Figure

3, it can be seen that one component of limit accuracy is the

VBE mismatch between the QX6 transistors, and the QX5

transistors. If the transistors always ran at the same current

level there would be no VBE mismatch, and no contribution

to the inaccuracy. The QX6 transistors are biased at a

constant current, but as described earlier, the current

through QX5 is equivalent to ILimit . VBE increases as ILIMIT

increases, causing the limited output voltage to increase as

well. ILIMIT is a function of the overdrive level

((AV x VIN - VLIMIT) / VLIMIT), so limit accuracy degrades as

the overdrive increases (see Figures 28 and 29). For

example, accuracy degrades from -20mV to +30mV when

the overdrive increases from 100% to 200% (AV = +2,

VH = 500mV).

Consideration must also be given to the fact that the limit

voltages have an effect on amplifier linearity. The “Linearity

Near Limit Voltage” curves, Figures 30 and 31, illustrate the

impact of several limit levels on linearity.

Limit Range

Unlike some competitor devices, both VH and VL have

usable ranges that cross 0V. While VH must be more

positive than VL, both may be positive or negative, within

the range restrictions indicated in the specifications. For

example, the HFA1115 could be limited to ECL output

levels by setting VH = -0.8V and VL = -1.8V. VH and VL

may be connected to the same voltage (GND for instance)

but the result won’t be a DC output voltage from an AC

input signal. A 150mV - 200mV AC signal will still be

present at the output.

Recovery from Overdrive

The output voltage remains at the limit level as long as the

overdrive condition remains. When the input voltage drops

below the overdrive level (VLIMIT/AV) the amplifier returns to

linear operation. A time delay, known as the Overdrive

Recovery Time, is required for this resumption of linear

operation. Overdrive recovery time is defined as the

difference between the amplifier’s propagation delay exiting

limiting and the amplifier’s normal propagation delay, and it

is a strong function of the overdrive level. Figure 32 details

the overdrive recovery time for various limit and overdrive

levels

Benefits of Output Limiting

The plots of “Pulse Response Without Limiting” and “Pulse

Response With Limiting” (Figures 4 and 5) highlight the

advantages of output limiting. Besides the obvious benefit of

constraining the output swing to a defined range, limiting the

output excursions also keeps the output transistors from

saturating, which prevents unwanted saturation artifacts

from distorting the output signal. Output limiting also takes

advantage of the HFA1115’s ultra-fast overdrive recovery

7

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