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LT5514

5514f

APPLICATIO S I FOR ATIO

Clipping Free Operation

The LT5514 is a class A amplifier. To avoid signal distor-

tion, the user must ensure that the LT5514 outputs do not

enter into current or voltage limiting. The following discus-

sion applies to standard mode operation at maximum gain.

To avoid current clipping, the output signal current should

not exceed the DC quiescent current, IOUT = 40mA (typi-

cal). Correspondingly, the maximum input voltage,

VIN(MAX), is IOUT/gm = 133mV (peak). In power terms,

PIN(MAX) = –10.8dBm (assuming RIN = 108Ω).

To avoid output voltage clipping (due to LT5514 output

stage saturation or breakdown), the single-ended output

voltage swing should stay within the specified limits; i.e.,

≤ VOUT ≤ 8V. For a DC output bias of 5V, the maximum

single ended swing will be 3Vpeak and the maximum

differential swing will be 6Vpeak. The simultaneous onset

of both current and voltage limiting occurs when ROUT =

6Vpeak/40mA =150

Ω (typ) for a maximum POUT =

20.8dBm. This calculation applies for a sinusoidal signal.

For nonsinusoidal signals, use the appropriate crest fac-

tor to calculate the actual maximum power that avoids

output clipping.

For nonoptimal ROUT values, the maximum available out-

put power will be lower and can be calculated (considering

current limiting for ROUT < 150Ω, and voltage limiting for

ROUT > 150Ω). The result of this calculation is shown in

Figure 7.

The LT5514 input should not be overdriven (PIN >

–10dBm). The consequences of overdrive are reduced

bandwidth and, when the frequency is greater than 50MHz,

reduced output power.

Input Bias Voltage

The LT5514 IN+, IN– signal inputs are internally biased to

1.48V common mode when enabled, and to 1.26V in

shutdown mode. These inputs are typically coupled by

means of a capacitor or a transformer to a signal source,

and impedance matching is assumed. In shutdown mode,

the internal bias can handle up to 1

µA leakage on the input

coupling capacitors. This reduces the turn-on delay due to

the input coupling RC time constant when exiting shut-

down mode.

If DC coupling to the input is required, the external

common mode bias should track the LT5514’s internal

common mode level. The DC current from the LT5514

inputs should not exceed IIN(SINK) =–400µAandIIN(SOURCE)

= 800

µA in Standard mode and half of these values in Low

Power mode.

Stability Considerations

The LT5514’s open-loop architecture allows it to drive any

practical load. Note that LT5514 gain is proportional to the

load impedance, and may exceed the reverse isolation at

frequencies above 1GHz if the LT5514’s outputs are left

unloaded, with instability as the undesirable consequence.

In such cases, placing a resistive differential load (e.g., 2k)

or a small capacitor at the LT5514 outputs can be used to

limit the maximum gain.

The LT5514 has about 30GHz gain-bandwidth product.

Hence, attention must be paid to the printed circuit board

layout to avoid output pin to input pin signal coupling (the

evaluation board layout is a good example). Due to the

LT5514’s internal power supply regulator, external supply

decoupling capacitors typically are not required. Likewise,

decoupling capacitors on the LT5514 control inputs typi-

cally are not needed. Note, however, that the Exposed Pad

on the LT5514 package must be soldered to a good ground

plane on the PCB.

PGA Function, Linearity and NF

As described in the Circuit Operation section, the LT5514

consists of a variable (step) attenuator followed by a high

Figure 7. Maximum Output Power as a Function of ROUT

ROUT (Ω)

100

1000 2000

5514 F07

STANDARD MODE

LOW POWER MODE

VCC = VCCO = 5V

CURRENT

LIMIT

VOLTAGE

LIMIT

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