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. A straightforward and cost-effective solution is a 2-way 180° power splitter. Such a splitter is 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 the UHFLI and the ZFSCJ-2-1 2-way 180° power splitter from Minicircuits: the results are described in this post.
Applications
- Compensation of noise correlation in laser setups
- Differential amplification of a DUT signal vs a reference signal
- Dynamic offset compensation
Figure 1: Schematic of the differential amplifier setup with the lock-in amplifier.
Figure 1 shows the setup. A differential signal is applied to Port 1 and Port 2 of the power splitter. The drivers of this signals should be 50 Ω 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, which has an internal low-noise amplification stage. Alternatively, the signal can be amplified using a 50 Ω compatible voltage amplifier, such as the HVA-500M from Femto Messtechnik GmbH.
Figure 2: Differential signal as applied to Port 1 and Port 2 of the splitter (left) and output signal on Port S (right).
A power splitter is usually based on a transformer-type implementation and does not add noise to the system. However, as seen in Figure 2, there is a small loss in the splitter. In the example above a 2*600 mV=1.2 Vpp differential signal is converted into a 800 mV single-ended signal. This corresponds to a 3.5 dB 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 > 30 dB up to 500 MHz as shown in Figure 3. (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.