Design of Low-Temperature and Radiation-Hardened JFET Direct Coupled Op-Amps without Current Mirrors

Abstract

The offset voltage (Voff) of the BJT and CMOS two-stage operational amplifiers (Op-Amps) substantially depends on the numerical values (differences from unit) from the current ratio (Ki≈1) of the current mirrors (CM). The CM parameter is also influenced by the Early voltage of their dominant active components. For the JFET technologies (Si, SiC, GaAs and others) with a low noise level, there are no high-quality CMs of this class today, or their construction (at Ki=1) is associated with a significant deterioration of other parameters of the Op-Amp. Nowadays, the current JFET mirrors are the weakest link in modern JFET analog circuitry and it is impractical to use them in the structure of the JFET Op-Amps. For the first time the article poses and solves the problem of determining the conditions for exclusion of the CMs in the JFET Op-Amp for the case when it is necessary to obtain a small Voff. It is shown that for this, three identical reference current sources should be used, which are implemented on the JFET transistors and the local negative feedback resistors. The Voff of the Op-Amps with the classical and proposed architectures are compared. The computer simulation results of the offset voltage (Voff) in the LTspice environment are presented, which show that silicon JFet direct coupled Op-Amps without CM have a systematic component of Voff at the level of tens to hundreds of microvolts and voltage gain of more than 80 dB in a wide temperature range.