A(V/V) = K x

x

RF

RG

1

1 + e

N - VG

VC

LMH6504

SNOSA96D – NOVEMBER 2003 – REVISED MARCH 2013

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As the IRG_MAX limit is approached (with increasing input voltage or with lowering of RG), the device harmonic

distortion will increase. Changes in RF will have a dramatic effect on the small signal bandwidth. The output

amplifier of the LMH6504 is a current feedback amplifier (CFA) and its bandwidth is determined by RF. As with

any CFA, doubling the feedback resistor will roughly cut the bandwidth of the device in half. For more about

CFA’s, see the basic tutorial, OA-20, “Current Feedback Myths Debunked” (SNOA376), or a more rigorous

analysis, OA-13, “Current Feedback Amplifier Loop Gain Analysis and Performance Enhancements” (SNOA366).

OTHER CONFIGURATIONS

1) Single Supply Operation

The LMH6504 can be configured for use in a single supply environment. Doing so requires the following:

a. Bias pin 4 and RG to a “virtual half supply” somewhere close to the middle of V

+ and V-range. The other end

of RG is tied to pin 3. The “virtual half supply” needs to be capable of sinking and sourcing the expected

current flow through RG.

b. Ensure that VG can be adjusted from 0V to 2V above the “virtual half supply”.

c. Bias the input (pin 2) to make sure that it stays within the range of 1.8V above V-to 1.8V below V+ (see “Input

voltage Range” specification in the Electrical Characteristics table). This can be accomplished by either DC

biasing the input and AC coupling the input signal, or alternatively, by direct coupling if the output of the

driving stage is also biased to half supply.

Arranged this way, the LMH6504 will respond to the current flowing through RG. The gain control relationship will

be similar to the split supply arrangement with VG measured referenced to pin 4. Keep in mind that the circuit

described above will also center the output voltage to the “virtual half supply voltage”.

2) Arbitrarily Referenced Input Signal

Having a wide input voltage range on the input (pin 2) (+/-3.2V typical), the LMH6504 can be configured to

control the gain on signals which are not referenced to ground (e.g. Half Supply biased circuits, etc.). We will call

this node the “reference node”. In such cases, the other end of RG (the side not tied to pin 3) can be tied to this

reference node so that RG will “look at” the difference between the signal and this reference only. Keep in mind

that the reference node needs to source and sink the current flowing through RG.

Application Information

GAIN ACCURACY

Gain accuracy is defined as the actual gain compared against the theoretical gain at a certain VG (results

expressed in dB) (See Figure 43).

Theoretical gain is given by:

(3)

Where K = 0.965 (nominal) N = 0.96V & VC = 80mV @ room temperature

For a VG range, the value specified in the tables represents the worst case accuracy over the entire range. The

"Typical" value would be the worst case difference between the "Typical gain" and the "Theoretical gain". The

"Max" value would be the worst case difference between the actual gain and the "Theoretical gain" for the entire

population.

GAIN MATCHING

Gain matching as the limit on gain variation at a certain VG (expressed in dB) (see Figure 43) and is specified as

"Max" only (no "Typical"). For a VG range, the value specified represents the worst case matching over the entire

range. The "Max" value would be the worst case difference between the actual gain and the typical gain for the

entire population.

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