GHz Differential Amplifier using a 180° 2-Way Power Splitter

Every now and then, a differential amplifier in the frequency range of the UHFLI lock-in amplifier is needed. The requirements for such an amplifier are low noise and a frequency range of at least 600 MHz. A commercial solution with an active differential amplifier is not available today.

An easy-to-use and cost-effective solution is a 2-way 180° power splitter. Such a splitter is usually used to generate a differential signal from a single-ended signal. But, it can also be used in the reverse way to generate a single-ended signal from a differential input.

We tested an example setup with UHFLI and the ZFSCJ-2-1 2-way 180° power splitter from Minicircuits, the results are described here.


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Figure 1: Schematic of the differential amplifier setup with the lock-in amplifier.

The figure above explains the setup. A differential signal is applied to the Port 1 and Port 2 of the power splitter. The drivers of this signals should be 50Ω compatible, otherwise the signal quality can be significantly lowered. The resulting single-ended signal is available on Port S of the power splitter (see figure below). The signal can now be directly connected to the UHFLI lock-in amplifier, which has an internal low-noise amplification stage. Alternatively, the signal can be amplified using a 50Ω compatible voltage amplifier, such as the HVA-500M from Femto Messtechnik GmbH.


Figure 2: Left: Differential signal as applied to the Port 1 and 2 of the splitter. Right: Output signal on Port S.

A power splitter is usually based on a transformer-type implementation and does not add noise to the system. However, as seen in the figure above, there is a small loss in the splitter. In the example above a 2*600mV=1.2Vpp differential signal is converted into a 800mV single-ended signal. This corresponds to a 3.5dB loss in the splitter.

A further important aspect is the phase unbalance and the common-mode rejection of the splitter. The phase unbalance is specified in the datasheet of any power splitter. In the case of the ZFSCJ-2-1 it is approximately 180°+/-1°. The common-mode rejection ratio (CMRR) of the differential input ports 1 and 2 is >30dB up to 500 MHz as shown in the figure below. (The CMRR is the relation Adiff/Acm of the differential gain vs the gain of a common-mode input signal in dB)



Figure 3: Common-mode rejection of the power splitter.