HTS dc SQUID based rf ampliﬁer: development concept G.V. Prokopenko a, * , S.V. Shitov a , I.V. Borisenko a , J. Mygind b a Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Mokhovaya Street 11, 103907 Moscow, Russia b Department of Physics, Technical University of Denmark, Lyngby, DK-2800, Denmark Abstract We present a concept of a rf ampliﬁer based on a directly coupled dc SQUID with bicrystal junctions, which have high saturation power and can be used with SIS mixers or possibly for satellite and cellular phone communications. A novel input resonant circuit is proposed using single layer of HTS. Estimated parameters are (per stage): central frequency  11 GHz, bandwidth  400 MHz, noise temperature  10 K, gain  10 dB and input saturation  1000 KGHz. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: HTS Josephson junction; dc SQUID; rf ampliﬁer 1. Introduction According to recent tests [1] a low-noise dc SQUID based rf ampliﬁer (SQA) made from low- T c materials can have quite low saturation power (of order of 100 K or even lower). It is possible to associate this with low characteristic voltage V C ¼ I C R N (where I C is the critical current and R N is the normal resistance). In Ref. [1] we studied the dynamic range of LTS SQA as a dependence of gain on temperature of the saturating input noise that is normalized to 1 GHz bandwidth: T 1 GHz SAT  V 2 B =ð8R D k B 10 9 G SQA ), where V B is the optimum bias voltage for SQA noise temperature, R D is the dy- namic resistance, k B is the Boltzman’s constant, G SQA is the power gain of SQA. The dynamic range of SQA is limited by the ‘linear’ branch of its I –V curve that can be characterized by V C , which does not practically exceed 250 lV for our low-T c 1 lm 2 area junctions. To increase the saturation power, one needs junctions with higher V C . The characteristic voltage of HTS junctions can be as high as 8–10 mV at 4.2 K [2,3]. This is why the dc SQUID rf ampliﬁer made from high-T c materials can be a very promising device for a number of practical low-noise applications where small size and extremely low power dissipation are impor- tant. 2. Saturation power The optimum bias voltage can be estimated as in Ref. [2]: V B ðI B R N =2Þð1 ðI C =I B cosðpU B =U 0 ÞÞ 2 Þ 0:5 , where U 0 and U B are the ﬂux quantum and the optimum ﬂux correspondingly, U B  U 0 =4, and the current bias is I B  I C . Therefore, we can estimate the optimum bias voltage as V B  I C R N =2  V C =2 that is indeed proportional to V C . Physica C 368 (2002) 153–156 www.elsevier.com/locate/physc * Corresponding author. Fax: +7-095-203-8414. E-mail address: georgy@hitech.cplire.ru (G.V. Pro- kopenko). 0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0921-4534(01)01157-1