The preamplifier provides voltage gain (e.g., from 10 mV to 1 V) but no significant current gain. The second amplifier is typically a power amplifier (power amp). In an audio system, they are typically used to amplify signals from analog sensors to line level. Three basic types of preamplifiers are available: According to Friis's formula, when the gain of the preamplifier is high, the SNR of the final signal is determined by the SNR of the input signal and the noise figure of the preamplifier. The noise performance of a preamplifier is critical. It is used to boost the signal strength to drive the cable to the main instrument without significantly degrading the signal-to-noise ratio (SNR). An ideal preamp will be linear (have a constant gain through its operating range), have high input impedance (requiring only a minimal amount of current to sense the input signal) and a low output impedance (when current is drawn from the output there is minimal change in the output voltage).