What are the requirements? Integration of STOs into conventional RF circuitry means that the STO has to deliver 0dBm to drive for instance RF blocks such as a mixer.
Using magnetic tunnel junction devices we have seen that the output signal can be enhanced to -40 dBm (see section on Spin Valves vs Tunnel Junctions). This is already a considerable enhancement as compared to spin valves, but is still too low for device integration. The signal level can be further enhanced upon using an amplification of 20 or 30 dB that can be achieved without scarifying the noise level.
What are the challenges ? In our laboratory experiments we usually use an amplifier before registering the signal on the spectrum analyzer. This signal is then analyzed to estimate the integrated power. The problem with this measurement (as with the estimate presented in the previous section) is that there is a considerable impedance mismatch between the amplifier (or the measurement chain) which is at Z=50 Ohm and the STO, which is of the order of 100 to 1000 Ohm in the case of tunnel junctions. This impedance mismatch means a loss of output power.
High impedance broadband amplifier on CMOS The output power can be enhanced by about 10 dB when using a high impedance amplifier (the 10 dBs do not consider the additional gain of the amplifier itself). Such an amplifier is not commercially available, but can be manufactured on a 130 nm CMOS process. This has been done in 2008 by the research group of DACLE who have conceived, developed and tested a high impedance differential amplifier that is composed of three stages respectively, see Fig. 1:
- a high impedance entrance to extract the full voltage signal
- a voltage amplification stage of 22 dB
- a conversion stage to match the output to 50 Ohm.
Output power measurements have been performed on two hybrid configurations (STO chip and amplifier chip each of them wire bonded to the same platform) to demonstrate the enhancement of the output power. In the first configuration the STO is buffered by a standard 50 Ohm input amplifier and in the second configuration by our dedicated high input impedance amplifier. Measurement results are plotted versus oscillation frequency (Fig. 2).
In both cases the STO output power is computed from power measurements at the amplifier output combined with the amplifier gain values. From Fig. 2 it is clear that the impedance matching allows a gain of 6 to 11 dB on the output power, depending on the frequency, thus improving the “power issue” that is inherent to the STOs.
 IEEE JOURNAL OF SOLID-STATE CIRCUITS 45, 214 (2010), P. Villard et al.
Main contributors :
- Patrick Villard, Pierre Vincent, Jérôme Prouvée, conception, design and test of high impedance amplifier
- Hichem Louazani, Postdoc, design of high impedance amplifier
- Jean-Philippe Michel, characterization of output power using high impedance amplifier
- Dimitri Houssameddine, PhD STO microwave characterization
- Marie-Claire Cyrille
- Ursula Ebels